The DT 990 600 Ohm Premiums that I have blow the COPs out of the water. The bass is more accurate, more abundant and punchier. The highs and mids are also better. The soundstage is where they are unquestionably better than the COPs, as any open headphone is compared to closed. The built quality and comfort is better on the DT 990s too, assuming you get the premium version. But these headphones are hard to drive, can't be used without a good DAC and amp and aren't portable. Plus they're open, which always isn't convenient, even if it gives better sound in my opinion.
The COPs are extremely versatile. They're built very well for the price, especially compared to Monster and Beats by Dre products. The sound is adjustable, and they can be used with almost all devices, although they can still benefit from some basic amplification, not so much a DAC. They are much more portable, and sound great for a closed headphone. For $160 they're a bargain. For $200 they're a good deal. For $250 you can do better.
The sound slider is sort of a gimmick though, so don't base your purchase entirely on that. On positions 1 and 2, there's essentially NO bass. On position 4 the bass gets muddy and eats into the highs and mids. Position 3 is the sweet spot for me. Others may obviously disagree.
Anyway, that's my two cents. It's not fair to compare these to the DT 990 and DT 880 because they are open/semi-open and meant for home use while these are closed and meant for portable use. It would probably be beneficial to compare these to the DT 770 though, but I don't have those. Maybe someone else that does can chime in.