I was curious about what Mcandmar's upgrades sound like. So I applied his methods into my S.E.X 2.1 as well.
1. 100 ohm resistors to c1-c3, c2-c3
2. replaced 680 ohm resistor with C3X (I grounded its body to 13 and 23 since its supporter is plastic)
and cap rolling.
3. rolled capacitors with Mundor Supreme (sorry guys. This only make sense to me with price/performance ratio; even this is sketchy for me. If there are some caps make sound so good that these bring tears to me eyes, then I might try haha).
In case, there is mystical break-in in any components, I played some music for about 10 hours. And I listened to it about 30 hours or so until now.
Overall I like how it sounds; treble is little more refined, mid is little smoother, and bass is little deeper and puncher. Also overall instrument separations and image have been improved. I did all of them in one shot, but I believe it is cap that makes sounds better since 1 and 2 is nothing to do with sound improvements. (Maybe lower noise sounds did some trick as well haha)
Did it make the amp quieter? Yes, it did. However, it is not so much that I can use with my IEMs. Putting a 75 or 150 ohm adapter is still needed in order for me to listen with any IEMs comfortably I guess this is same as putting 120 ohm resistors into headphone jack, but it is not a permanent method. But maybe my tubes are nosier than others.
Are those worth? 4 Mundor Supreme caps costed me $60 from ebay, 2 C3X and 2 resistors were $30 from Mouser, 3d printed supporters were free from my boss. So it is about $90 (included s&h) upgrade. If you are satisfied with S.E.X 2.1, then no. If you want to experiment (since it is a S. E. eXperimenter model after all) with your amp or want to squeeze its potential, and you have an access to 3d printer (I think there is a company prints out 3d parts online), then yes, it is worth. And I'm in the 2nd option.
It was very fun for me to upgrade parts and I'm very satisfied with the result.
Thanks Mcandmar!