that seems like exactly what I have set up except I gave my DT990 pros to my brother and got the 600 ohm version. I would say the compass is pretty good. Definitely a nice step up if you don't have anything right now. I would say go for a darkvoice amp or the audiotailor jade. The good thing about the jade is that it's relatively cheap and gives you different sound signatures depending on the socket and gives you a lot more power than the compass. If you get audiotailor, a nice DAC might be the DACmagic if you can find one for a little over 300 used. That would end up with ~650 for both.
The option above is assuming you get both the DT990 600 ohm and hd650. However, if you plan on just getting the hd650, the compass would not be a bad choice to just combine both. The only reason I got jade is to keep me from buying anything crazy before my friend finishes my b22 and to give the extra power my DT990 craves. So if you're planning on getting both for the long run, definitely go for tube amp + dac because the compass might leave you wanting more. I don't mean to discredit the compass in any way, shape, or form, but my DT990/600 definitely do much better on the jade. the HD650 sound great on the jade too.