I've spent a decent enough of time with the T1 throughout the years. If there's lots of 10khz content (i.e. cymbals), its painful to listen to for a long period of time. If there's a lot of bass and know what good bass sounds like, you might notice the cups resonate which is not surprising when the T1 is basically a driver into a
hollow plastic cup. Even if none of these issues existed, the treble just doesn't sound right.
Unlike the DT880, the T1s I've listened to don't sound very consistent between samples nor do they actually sound a whole lot better then the DT880s besides a better soundstage. No, I'm one of those people who don't equate boosting the treble as increasing headphone resolution and I don't think the T1 resolves appreciably better than the DT880.
Really, headphones that can't render all types of music well and are very DAC/amp dependent are headphones with problems. This is literally a non-issue in the world of speakers (i.e. I do not think there is a single sensible person who believes good full range speakers should not be able to play all genres of music extremely proficiently) so why should it be the case for top of the line headphones?
I'm sort of trying to turn you away from the T1 V2 because if you're paying $1,200 USD for a pair, you're doing the equivalent of purchasing a FX series AMD CPU in this day and age. If you don't know any better, you probably end up thinking its just as good as an Intel processor...except it really isn't. If you must have the T1, you can probably do a better job than Beyerdynamic by buying the T1 V1 for significantly cheaper and loading the plastic cups yourself with dynamat and cotton. I don't really believe for a second that they've made radical improvements with the T1 V2, considering they cared so little that they couldn't get driver matching right on their kilobuck headphones.
About the amps: I don't really don't need to listen to the A20 to know its nothing special when stuff like the Bottlehead Crack + Speedball, ECP Torpedo, those Meier Audio amps with hardware crossfeed, like the entire Schiit Audio collection, and Cavalli Liquid Carbon are in the same price range. And that's just some off the stuff from the top of my head. If there was any single thing remotely special about it you'd have far more people buying it...yet no one really talks about it. Really, the only special thing about it is that it uses a 100 ohm output impedance to impart its "warmth".