I also would like to ask this question, though my reading on the forum has given me only a couple of good options, among which it seems that the Schiit Valhalla is apparently quite good: http://schiit.com/products/valhalla as a tube amp, which according to someone was designed partly with the dt990 in mind. It's $350 in the US, but probably quite a bit more elsewhere.
I'm trying to chose between the Lehmann Rinelander, and the Heed Canamp, as well as the Musical Fidelity v-can Mk ii. They are all solid state, meaning no tubes, and vary in price respectively, about 550, 400, and 150. Supposedly, none of them are bad, but I'm not sure about how they actually sound with a music streamer ii and the dt990 pros. (I'm in Israel, which is why some of my products are already considered old, or discontinued. As far as I know, discontinued, doesn't mean bad, just that for instance in the case of the v-can, they have newer products out now.)
I have the Schiit Magni, but I think it's a bit harsh.