I don't actually have the HD 600/6XX, but was just wanting to know if they compliment each other well, one more bassy, one more neutral, because i might get the two. Thanks for the reply though!
The HD650 and the 99C are more similar than different imo. Both have a warm, thick sound, a bit of roll of in both ends, a bit exaggerated midbass and lovely midrange presentation. They're certainly not identical sounding but more similar than many other headphones that I've heard. I think that it's a fair chance that a person who enjoys one of them will also enjoy the other if that makes sense.