The best T1 that I heard sounded dark... with some sibilance spikes that were more or less the same as I could remember from my DT880/O2 combo... and it was extremely thick and warm, with a lot of sub bass rumble. Overall, I guess it could be considered an upgrade to my old DT880 600... all else considered. And yeah, Daniel would have cringed just looking at the headphone.
Anyway, this DT880's bass is not really a problem. My amp is already bringing it up to par with my ES10, and I could always implement an extra bass boost circuit to match the HE-560. The overall sound of this DT880 is very nice, though. I am actually enjoying it over my other headphones in some cases where the extra extended treble and upper midrange of them become a problem.
As for the ES10, I've since then improved it by quite a lot. It's interesting that you'd think they are Stax-like, since I guess speed and clarity were the focus of my earlier mods. Right now it's all about the overall dynamic. I just want it to sound balanced between relaxing and exciting.
Bass being that overblown is actually a necessity in mobile environment as bass often gets drowned out by other things. That's why I set out to create an amp in the first place. What I have ended up creating up until this point is simply amazing IMO. Hopefully you'll get to demo the pairing with the DT880 at the next meet, because I am thinking this may just be one of the best pairings for the DT880 yet... if the truth is really that my DT880 has only sounded this good due to the amp.
Haven't really heard a stock Tesla headphone that I'd consider dark...Those are the ones that I would truly call sibilant.
I've long since changed from being a basshead. It just doesn't feel clean when you take a bassy headphone and pair it with bassy music. Bass Boost isn't terrible to implement, so could definitely be an option for you to consider for your amp. Also, clean up that circuit! That wiring made the inner EE in me cringe. haha
I thought the ES10s sounded pretty impressive. Not sure how much of it is due to the custom cups, since those are pretty hard to source given how busy Martin is. I would imagine the internal dampening scheme makes the biggest difference. The driver in the ES10s are pretty impressive, similar to their full-size range if I'm not mistaken. (don't quote me on that!) Have you heard the ES700? With both headphones stock, I would take ES700 over ES10, disregarding difference in headband build quality, but I'd take your modded ES10 over either.