Just tried both my T90 and my T70 on the A4 (both 250Ohm), on high gain it drives them to far higher levels than I'm willing to listen to. Volume setting at 12:00 (i.e. the marker sitting at the top, halfway through the range) is already much too loud. As I said: I don't see any problem using the A4 with higher impedance cans. It even drives the T1 with 600Ohm impedance to respectable levels, while a bit lower in volume than the 250Ohm headphones its still able to produce levels I'd never use for listening.
great. do you find combo bright?