It's not that they sound bad, they don't, technically. They sound almost like Q701's in terms of sound signature, detail, imaging, sound stage, etc. But the sound they produce is still too thin, too analytical, too hollow, without body, even less body than K701 in fact. Its what it says on the box: reference headphone. Its for studio use, for mixing, not for listening to music, and you can totally feel that while using them. I mean, comparing the experiance of listening to them to HD650's for example would be... HD650's are like drinking hot, sweet, thick cocoa drink, while K550's are like drinking ice cold carbonated mineral water.
The biggest problem for me, same as with Q701, but to even bigger extent, is sibilance. In some music its like listening to SSSSSSSSSSSSSSSSS all the time. Even on well recorded CDs...like Patricia Barber - Companion. Usually its a nice bassy warm sounding music, very cozy with HD650's, but with K550's sibilance dominates the experiance and it just lacks body. These are not as good for vocals or jazz as Q701's. I find they sound best with acoustic guitars.