That's a tough one... If both are properly amp'd it's kind of a matter of personal preference between the DT 990 & Q701, and you did say you were loving the former even without an amp no?
You could try getting an amp like the Magni ($100, cheaper than a new pair of headphones) in order to figure out what you may be missing right now, it should absolutely make a difference for the Beyers... But if for some reason it disappoints you can just return it and then look at other headphones/options (tho realistically the Q701 need an amp to play optimally as well).
I'm assuming you'd keep the mixamp either way since you already have it, the way your post's worded makes it seem like you'd only have it with the 701's.
Edited by Impulse - 2/28/13 at 2:18am