Thanks everyone for all your help with capacitor upgrade options / discussions, etc. I posted all my thoughts on both the S.E.X. and Mainline upgrades over at Head-Fi, but here's what I wrote in case you don't want to have to split time across 2 forums:
S.E.X Attenuator & Capacitor Upgrade - May 2014
During May 2014 I began to modify my S.E.X. and Mainline amps beginning with the volume pot in the S.E.X. which I replaced with a stepped attenuator, assembled using high quality Dale Vishay resistors. I chose this particular attenuator because the Dale resistors are known to be very transparent and are actually the same as those used in the Mainline kit. By all accounts, you can achieve similar results with other options - the key is transparency.
Attenuator Installation
The first thing to note if you are changing your attenuator (or potentiometer) is that you might need to carve out a bit of extra space from the locking channel where the pin / tab of the attenuator sits into the chassis plate in order to prevent it spinning. Thankfully, the aluminium chassis plate is easy to work with using either a hand-file or a dremel. In my case I had to lengthen and slightly enlarged the stock groove linked to the hole for the volume pot in the chassis plate. This isn't visible from the top once the volume knob is in place so don't stress if it's not the tidiest job ever (so long as you don't overshoot the width of the volume knob, but that would take some extreme carelessness).
Once the hole is ready, mounting the new attenuator is as easy as the stock one. Remove the supplied nut and washer(s), insert the spline through the hole, align the locking tab / pin and tighten the nut from the top side of the chassis. That's it for mounting - simple! The next step is wiring and that can change depending on the model of attenuator you choose so please refer to the provided instructions for your attenuator of choice and the instructions from the S.E.X. kit. If you get stuck, the wonderful people on this forum can also usually help.
Do be aware at this point though that you may find the stock volume knob sits a little higher than it used to. It's not a major drama, but for the aesthetically inclined, you might need to modify the stock knob or track down a different one if you want the knob sitting flush against the chassis.
Attenuator Sound Improvements
You're not going to make a change like this unless there's a payoff so let's discuss what happens when you change the attenuator. The exact changes may vary depending the brand / model used, but if you go for something known for transparency (like I did) then you'll get a nice increase in the detail and clarity of your S.E.X. The signature of the amp doesn't change significantly other than perhaps a slight bit more brightness, but it's subtle enough that I'd say to expect no real change in signature and just an increase in detail retrieval and texturing of sounds.
As an upgrade, I would say it is absolutely worth it. Including the time taken to modify the hole in the chassis I would say this is a 1-2 hour project and delivers a sound upgrade second only to the C4S mod. You can complete this mod on a budget of about $50 USD, but can also spend a bit more for more exotic attenuators if you want to.
Capacitor Upgrades
Next on my list was a double-barrelled capacitor upgrade. I approached the upgrade in 2 stages at the excellent advice of either JamieMcC or mcandmar - I apologise for not remembering which one gave that specific advice, but they both deserve kudos for their input and support over the course of multiple mod discussions around capacitors and the like. The reason for the 2-stage approach was to isolate the impact of each pair of capacitors, but the problem was that the sound was still limited by other caps in the system so I am going to summarise the whole upgrade in one hit because I can't truly describe the full potential of the FT-3 teflons I installed first without having upgraded the other (output) caps which then brought their own influence to the sound. Hopefully that makes sense...
I started off installing some FT-3 0.1uF teflon capacitors (the same as the ones used in the Mainline upgrade below) and immediately noticed improvements even though they require significant burn-in. These caps replaced the pillow-shaped brown / orange capacitors that sit over the tube sockets. Early improvements I noticed were cleaner, clearer textures in the sound and better layering of instruments in the soundstage (i.e. different instruments were now more clearly defined 'in space' within the soundstage). As I said above though, the full impact of the teflon caps couldn't reveal itself because they were hidden behind the stock output caps (which are good, but able to be improved).
Stage 2 was the installation of output capacitors - the ones that connect the sound to your ears (kind of). I chose the Mundorf Supreme Silver / Gold / Oil caps for this purpose after doing a lot of reading about this particular model's excellent warmth and midrange. I decided that the Mainline should stay as neutral as possible while the S.E.X. would become my fun, organic, musical amp. Don't get me wrong though - this doesn't mean I was trying to turn the S.E.X. into some kind of thick, syrupy mid-monster. I wanted musical, enjoyable sound that was still nimble, agile, liquid and accurate. The good news is that the mods were a resounding success.
With the Mundorfs installed, the S.E.X. instantly took another small step in a very positive direction. All of the sound was warmer and fuller, but not at all heavy or thick. Music is still well-paced and lively, but with a little more note weight (not quite as good as the Mainline in this regard, but closer) and more present mids.
The staging from the S.E.X. is still excellent, but the Mundorfs added to what I was already hearing from the FT-3 teflons by helping to deliver greater fine detail and textural cues.
No doubt there are even more (subtle) changes that took place as a result of my upgrades, but the inability to truly A/B makes a direct and detailed comparison impossible. What I can tell you by way of a summary is that the S.E.X. post-upgrade does everything good that I wrote about above only now slightly better and with a dash more soul and warmth. I imagine you could get similar improvements with different cap combinations too and encourage you to give it a go. Changing these 2 caps on the S.E.X. is a relatively easy process and it's fun to hear what results.
Mainline Cap Upgrade Update
Over the past few weeks (April-May 2014) I've installed two sets of capacitors into the Mainline starting with a set of 18uF Auricap High Resolution capacitors in the position of the stock Dayton 10uF capacitors and a set of 0.1uF FT-3 teflon capacitors fresh from Romania replacing the stock 0.1uF Daytons on the Bias Regulator board.
FT-3 Teflons - 0.1uF 600V
I'm going to start this update with the teflons on the regulator board. I've literally just finished this installation and it may require some time for burn-in of the teflon caps, but I honestly can't hear any noticeable difference in the sound and would question the value given that the installation is a bit ugly and clunky due to the size of these capacitors. As you can see in the image attached I've had to run fly leads from the teflon caps which are cable tied to the Auricaps. I've also use some heat shrink tape to create an insulating buffer to keep the teflon caps sitting a little way off the Auricaps (even though there should be no issue with them touching due to the plastic coating on the Auricaps.
These FT-3s cost around $10 per pair plus postage (via eBay) so it's a small enough investment that I really don't mind if they have done absolutely nothing to the sound because they haven't taken anything away and if they provide improvements after burn-in then that's great.
I found some variation in the bias settings (adjustments to ensure identical voltages for each output tube) following the install so if you do change the regulator caps be sure to double-check the biasing if you want to maximise performance.
Auricap High Resolution Polypropylene - 18uF 400V
These have been in place for a few weeks now and are definitely a worthy upgrade. They cost me about $130 for the pair from a local supplier and are the best $130 I've spent in a while. As you can see in the picture, they just fit in the space between the C4S boards and the output transformers, but still need a tie down using an adhesive anchor on the chassis (the same as the ones supplied with the Mainline kit to tie down the Cat 6 cables).
If you've read my review of the Mainline on Head-Fi, you'll know that the treble on the Mainline in stock form is extended and detailed, but still smooth and non-fatiguing. I did say in the original review that there's no hint of fatigue when listening to the Mainline, but I have to call myself a liar now and say that there must have been the tiniest hint of harshness / graininess there. I know that because now it's gone and its absence has left all of the Mainlines magic on full display. Don't get me wrong, the stock Mainline kit is in no way flawed. In fact, as I see it, the ability to spend $130 and 15 minutes to bring the amp to another level shows just how good this circuit is.
The sound with the Auricaps is basically identical to the overall sound of the stock Mainline only smoother, more defined and just better. Treble is smoother while still fully extended and detailed. Midrange textures are sophisticated and intricate, and the bass is extended, textured and punchy. As I said above, the best thing about the Auricaps in the Mainline is that they don't change a great thing. They keep the Mainline's signature and neutrality completely intact and just let it do what it does better. It's hard to say exactly how much influence the Auricaps have had on this next part, but I would swear that the Mainline now has a clearer, stronger image than before. This is most likely due to the slightly smoother treble allowing all the other auditory cues to arrive cleanly and accurately, but the staging and imaging now with the Beyer T1s is really magical. It was good before - outstanding even, but it definitely seems even better to these ears now. I am having "wow" moments multiple times each day.
One other note as to why I chose the 18uF rather than a direct replacement of the 10uF stock caps. From the reading I did, a larger capacitance can provide better bass response which I was keen to explore to see if there was any room for improvement in the case of the Mainline. The results are that if there's an improvement there I can't tell. The hardest challenge here is that you can't quickly A/B a capacitor change like this so it's all based on auditory memory and the difference in bass extension (if there is one) is small enough to be insignificant, unlike the change in sophistication and smoothness in the mids and treble.
Capacitor Wrap
The only other caps in the circuit that could potentially benefit from upgrading are the electrolytics used on the power supply board, but they fit so neatly on the board and are already good quality Panasonic capacitors so the potential benefits here seem questionable (although I'm always open to being convinced otherwise).
If I were asked today what to upgrade in the Mainline, I would strongly urge building the stock kit first and then considering the Auricaps if you like the stock sound. If you yearn for something warmer or lusher - more tubey perhaps - then the Mundorf Supremes might be a better choice (write up of Mundorf Supreme Silver / Gold / Oil coming to the S.E.X. review soon), but I definitely think there's huge value in hearing the stock kit first so you know which way to tweak (or not tweak) the sound if you choose to upgrade the output (technically parafeed coupling) caps. Unless something changes in the coming days / weeks in which case I'll be back to edit this post, I wouldn't really recommend changing the regulator caps. Keep the build neat and simple.
