The Beyerdynamic DT 880, Beyerdynamic DT 990, and the AKG Q701 are all cheaper or equal in price to the Sennheiser HD 598 and all are considered a tier above the HD 598. The DT 880, DT 990, and Q701 are in the same tier as the HD 600/650. It's more about whether what headphone will suit is personal preference in sound the most for the price is looking at.
I don't know anything about the Logitech UE 6000 or the AKG K550, so I can't really comment on them. Honestly I think it's subjective whether the K550 or UE 6000 sounds better, it's a matter of finding your sound preference, which is hard to know unless you have tried multiple different pairs of headphones from different brands. Also Tyll on Innerfidelity or basically any reviewer are going by their preferences on headphones at least some, which may or may not fit your own preference. Finding the sound signature that suits you best is honestly a real pain. If I knew what sound signature I liked from the beginning I wouldn't of wasted so much money on headphones I never really liked. I wish it was easier to have the option to try before you buy on most of these headphones instead of going from reviewers that may or may not have similar sound preferences as you do.
Edited by kman1211 - 12/2/12 at 2:08pm