Well vs the Momentums:
The momentums seem to have a 10dB drop around the mids and treble, so the AKGs will definitely seem much brighter, but that doesn't mean that the sound bright. They sound bright in comparison to the Senns because the Senns have a severe lack of upper mids/treble it looks like ... If you look at the above graph, the brightest of them all, the AKG Q701, isn't even considered a bright headphone, I own both, as well as Grados SR80s and Sony V6s, Etymotics HF5 I think it is, the 550s to me they do not sound bright. They have much more of an 'AKG' flavor sound which I'd qualify as pretty neutral. If you read a few pages back, I was asking about their potential brightness issues, I heard none...
Also the bass of the K550 is
strong. It's not excessive but definitely more present than on the Q701s and quite solid bass (more so than Beyer 990s???), not a bass monster or pure basshead headphone, but I'd definitely qualify them as a warm sounding headphone with solid bass rather than a bright one. Definitely more of a warm inclination rather than bright imho. To say that they sound cold to me is rather incomprehensible...
As for the Momentums, before buying the 550s I did a little research and went 550 over the Momentums, seems like the Momentums are 350$ at amazon.com, and given the AKGs were almost half that price (paid 180$cdn for mine...), plus posts like these helped the decision become a really easy one:
http://www.head-fi.org/t/109756/rank-the-headphones-that-you-own/3225#post_9659760
http://www.head-fi.org/t/109756/rank-the-headphones-that-you-own/2610#post_9085066
http://www.head-fi.org/t/109756/rank-the-headphones-that-you-own/2475#post_8932059
http://www.head-fi.org/t/109756/rank-the-headphones-that-you-own/2445#post_8852286
http://www.head-fi.org/t/109756/rank-the-headphones-that-you-own/2175#post_8451388
But anyhow, these are positive 550s 'reviews', it depends what you're after. I was after an affordable neutral pair of closed headphones which would be an upgrade to the Sony V6 I've had for 10 years, and that they were... [edit] I see you had the V6s. To me the V6s are great, and they even give a decent run for their money to the 550s, and I'd say both are good buys at their relative prices (100$ for V6 & 200$ for 550s). But if you think the V6s have excessive treble, maybe you just prefer a sound with less treble... If that's the case, then yeah I think sticking to Sennheiser might be a good idea!