I listen to the Mainline exclusively with HD800 headphones, so I cannot comment on it's performance with IEMs. Also, I have to say that I am neither an expert on amplifier circuits nor an audiophile as experienced as Loquah. Still, I have tried something that worked out very well and might be helpful here:
If a lower output impedance ist what you are looking for, a possibility might be to use the Peerless/Magnequest TL-404 output transformer in place of the stock iron. Search the current Bottlehead froum as well as (both on Audio Asylum) the old Bottlehead forum and the Magnequest forum if you want more info about this device. There is a lot to read, the TL-404 has a somewhat legendary status. In theory, it gives you 5 different output impedances to choose from, namely 500, 200, 50, 10, 4 Ohms (with 5000 Ohm primary, which is not quite identical to the stock iron). Using the lowest impedance tap in conjunction with high sensitivity IEMs might even produce a lower noise floor compared to the stock iron (not that I ever had problems with noise from the amplifier, but just in theory if there was a problem with high sensitivity IEMs). Also, magnetic hum pickup from the power transformer can be a problem with the TL-404 and requires very careful placement, but this problem should also be smallest on the lowest impedance tap due to the lower number of turns on that winding.
Anyhow, some 3-4 years ago, shortly after I bought my Mainline kit, I bought a pair of TL-404s when over at the "All things Magnequest" sub-forum on the Bottlehead forum Doc B announced that Mike LaFevre would do a production run. That thread might be a good starting point on the subject. Be aware that a pair is 500-600 USD and that they are very scarce, since Mike does not seem to produce them very often. (I remember xcortes posting on the Magnequest forum some months ago that he has a pair or two to sell...)
Back to my experiences: I have listened to the Mainline for 2-3 years in stock form (only coupling capacitors were changed to 10uF Clarity CAP MR 630V somewhere along the way). A few months ago, when I unexpectedly had some free time, I decided to finally try the TL-404 in the circuit, "quick and dirty", using some clip leads. To my ears, coupled with the HD800, they offered a small but noticeable and worthwhile improvement in both temporal and spatial resolution. Actually, I liked the sound so much that I went all the way to permanently integrate them into the amp, with switches to choose between the different output impedances. Actually, turns out that I even got hooked on their sound in the months since ;-) (But, to be honest, I am not experienced enough to be 100% sure that it is all improvement that I am hearing and not in part some fancy coloration that I just happen to like.)
For all those who do not already own a Mainline, the stock kit already is divine and no tampering is required! Just buy one and be happily ever after :-) Still, for the special situation of the OP, the TL-404 might be a way to get lower output impedance and at the same time improve the sound a tiny little lovely bit.
I had promised in the aforementioned thread on the Magnequest sub-forum that I would do a write up about my experiences, and I will do so. I even already made pictures. But I want to take just some more months to be more sure of what I am hearing. But since I owe so much to Loquah's excellent reviews on HeadFi - they actually were the reason for me to buy the Mainline! - I would be happy if my preliminary report could be of at least a little help.
The picture below shows an impression from the TL-404 test in my Mainline, with one original transformer removed and the input of the TL-404 connected to the output from the parafeed capacitor via clip leads (circuit ground and chassis ground of course connected as well). In this particular experiment, the output of the 404 was connected to an AC voltmeter to measure magnetic hum pickup from the power transformer and thus determine the optimal position for the TL-404.
