Sorry for late reply, busy on my college stuff...
Well, I used usb otg with my OnePlus 3, and also Line in with Fiio X7 1st Gen, I never try on speakers. About their gain variables...for me, they changed quite large when turning up volume to High Gain, it's became much brighter and clear and clean in High Gain. I used AMPEREX 6DJ8 ECC88 tubes. Indeed, hybrids usually don't have much tube sound, but what I can say is changing tubes did help with the T1, I can notice AMPEREX 6DJ8 ECC88 changed the sound of T1 and much better than stock tubes. I think the DAC in T1 is their merit, they tuned their DAC much cleaner than WA7's ES9018K2M that I so hate to admit. I never try Little Dot before, so apologize that I can't really say anything how they compare to Little Dot. I tried MrSpeakers Aeon(13ohms) with it today, but only a while, as above mentioned, bright (a little too bright for me), clear, clean, and of course, the soundstage. Soundstage did Improved too but not with "Wow" effect, but they did made soundstage wider that you can noticed it significantly. I should mention that lower impedance doesn't meant it easy-to-drive, some Headphones like JVC HA-SW01, with general knowledge, on paper they doesn't feel hard to drive at all, (56ohm, 105db, 1mW), in fact, they are super hard to drive zzz.
Koss Gaming Headphone? LOL, i never try them before, I used ATH-M50x for gaming with E17k Alpen 2 (sometimes line out sometimes headphone out) to GoVibe Peak (you probably never heard of haha), the ATH-M50x is just fine for Gaming, their sound also very punchy and accurate for Gaming haha
My real question, though, is would this beat a Schiit Fulla 2 for music and games on HD598?