HD650 - I really enjoy the HD650/Crack combination. I think this is the first time I've had my HD650s properly amped and they sound fantastic. Very smooth, warm and full sound but with nicely detailed highs. Slightly darker (or veiled as some might describe it) compared to the Beyers? Yes, but much more natural and easy to listen to for long periods (too bad the comfort is not quite up to the Beyer level).
DT880-600 - There are things these do really well, like the clean and open top end and great soundstage which gives them more excitement than the HD650. But the bass strikes me as just a bit too lean and controlled, which has a tendency to push them towards the bright/harsh end of the spectrum. However, they sound the best I've heard them with the Crack which gives them a bit more bass and warmth (NOTE: mine are fairly new and may still need considerable break in).
DT990-600 - Only briefly listened to these on the Crack so far, but the bass struck me as way too emphasized. Yeah, these cans are known to have a U-shaped FR and that seems to really come through on the Crack. Bass way too in my face and top end somewhat confused compared to the DT880. On some other amps, I've actually preferred these to the DT880.
So far I find myself going back and forth between the HD650 and DT880 depending upon mood, music, etc. But any further listening will have to wait as I've disassembled the Crack to stain and finish the wood base.