two amplfiers i know of which are good...
audioengineD1 is $120 and is pretty good for the money.
schiit modi + magni (two seperate boxes... its a pair you buy both) are $100 and $110 and are also quite excellent for the price
of course if you wanted "virtual surround" as an option you could always buy a xonar dx and perhaps just use an external amplifier (without a dac in it) if you intended to get those 250ohm headphones. if you didnt want or need virtual then go ahead and get the external amp+dac combos as they are going to be a bit better sound.
if you liked what i described then perhaps they are best for you. another model to consider is the audiotechnica ad900x which is sometimes described in a similar light as the 990. please read one users comparison
I have some DT990's and after recently buying K550s, which are different but also very good, I decided to pick up a pair of Audio Technica A900x's. Like the K550's they are very good and open sounding for closed can, however, the signature is much more like the DT990's than the K550 is. The entire sound spectrum is really aggressive. I have A/B'd the A900x's and the DT990's extensively and the biggest difference is the extra few clicks in volume it takes to run the Beyers. I will actually go so far as to say there is a slightly higher bass quantity in the 900x's though the DT990 bass has more substance. The mids on the Beyers are just a tad richer and smoother, and on the AT's they are the slightly less recessed. The highs on the Beyers extend better, but can also be shrill sometimes. Overall the two cans are surprisingly similar in signature.