When Tyll initially tried and measured the COP he said he was quite impressed with the adjustable bass feature:
Supposedly later on when he reviewed the Tiesto he said something negative about the COP's adjustable bass but I can't find that now. If anyone has a link to that it would be appreciated.
IIRC the claim was that he said something negative about the COP's adjustable bass feature, not the overall quality of the COP's bass. This wouldn't be the first time something Tyll has said has been vaguely passed around these forums like a game of telephone eventually losing all resemblance of what he actually said or what he really meant. If you want to know how he feels about a particular headphone I recommend you read his words for yourself on innerfidelity.com where there's adequate context and he's not misquoted. He's quite involved in the comments section of his articles in case you have any questions for him.
@thatBeatsguy: sorry, I don't think I was very clear there. I have the T5p and absolutely LOVE them, but Tyll had made some pretty negative comments about them on a different forum (not on the IF website). Of course, that guy has forgotten vastly more about headphones than I will ever possibly know, so I said that with tongue firmly in cheek.
As for the COP, give them a shot!! I happen to be a big fan of the Beyer sound and while I haven't heard those specifically, there are definitely some people around who really like them. Tyll made a passing comment about them (here) during his review of the AKG 267 Tiesto headphone, saying "After hearing the Beyerdynamic Custom One Pros adjustable bass feature, I expected the AKG K267 Tiesto adjustable bass feature to be just another horrible sounding gimmick." This obviously leads to the conclusion that he was not a fan of the bass feature, but again, this doesn't mean someone else wouldn't like it. The only way to know would be to try them yourself, and if you don't have anywhere around to go and demo them, maybe you could pick up a pair used from the forums here or on eBay or something and minimize your loss if you do decide you'd like something else.
Personally, I'd give a 770 a go with EQ over the COP, particularly for monitoring. But that's not based on actually hearing the COP, just reviews and whatnots