Which headphones??

The 598s are supposed to be v. close to the 600s but that price seems a little high.

I have a dedicated headphone amp from a now infamous builder Singlepower. It is an OTL and sounds great, it is a pity that the builder/owner ripped a number of people off and disapeared from the scene. The Headphones I use with it is the AKG 701 which is detailed and spacious sounding. I recommend them as a good sound stager.

Check out these sites:
http://headfonics.com/
http://www.head-fi.org/
http://headphonereviews.org/
http://www.headphoneinfo.com/
http://www.headphone.com/
I can't stress enough the need to try each and any headphone you plan on using as they are very particular and unique to each and every set of ears. No two will sound alike to any two people you care to mention. Listener bias is the only deciding factor. For starters, get yourself down to the local Guitar Center or someplace similar and really listen to their demos. If you really want to torture yourself, go to any high end audio show and give a listen to the really good cans and they'll ruin you for keeping your budget low.

I thought I'd heard everything until I listened to some Audize cans through a Eddie Current tube headphone amp and only then did I understand what Srajan over at 6moons talks about when he describes performers as "lit up from within" and his particular way of describing musical events. It was a hauntingly beautiful experience and saddens me to know I'll never hear it again until the next show.

Good luck.
Nonoise

Senns are excellent phones; I've been using a pair of 600s for the past 15 years and they are sonically satisfying and comfortable. If you can find a good pair of used 600s or 650s you might want to consider.

Sennheiser 600 headphones and Bada hybrid tube amp. Very dynamic!