Elac Uni-fi UB5 and the Bower and Wilkins 685 s2's come to mind. Paired with a good amp and DAC, both really shine. Good value there, especially with the Elac's.
Regarding headphone amps and DACs, it's crazy to attest that they don't make a difference. Just go to your local audio store and try the range of amps and DACs there.
HD600s are awesome in my mind - I'm just waiting for them to be super cheap before I buy them. I have the HD650 and the HD595 (the older version of the HD598 - same drivers AFAIK). First, the HD595 and HD650 are pretty far apart in terms of sound quality, with the HD650s being much better. That's not to say the HD598s are bad! They are amazing, especially if you bought them during the holidays for $109. Are either the HD650s or HD600s worth 4x more than the HD598s? Depends on how much you value good music, but you're definitely in the 'diminishing returns' part of the figure if you got the steal of a deal on the HD598s.