The COP and DT990PRO are quite different cans.
If you stick to DT990PRO's v-sound signature, with deep base and sparkling treble, COP might not be your cup of tea.
And if you're looking for different type of sound or change of pace for DT990PRO, and if you like pretty bassy sound, COP will be very nice choice.
COP easily rivals DT990's in bass sound even at position 3 of custom slider.
And you need to realize COP, having closed back design, has rather narrower sound-stage compared to DT990pro, which has open-back advantage in sound-stage.
For your information.
This may be one of those times where I get em both <3 I Might go DT 990 Pro first since I do like the open soundstage and DEEP bass, and I wouldn't mind something brigter than the Dt 880 pro
While I'm looking to buy the Dt 990 Pro [B&H has the 250 ohm pro at $169 and Xb 1000 at the same time] so if the Sub bass on the the COP is nice it might substitute teh XB, because I like my Dub Step DARk and DIRTY all that matters is that BASS
so I'd like to have the XB 1000 but once I start working I can save for the Custom One to <3