Sennheiser HD600

i found the perfect mate that makes them sound beefy/warm and it works perfectly. i regret selling the set-up. i used the musical fidelity amp V2 (the v3 sucks by the way). i used the most expensive old mullards, and ps audio X-treme inter-connects. i promise this won't sound thin and much less then 1,000

..oh, by the way, the 600 are great, the 650 sucks--you have the right ones.

Maybe not the answer you're looking for, but I found with my 600s that upgrading the headphone cord itself made a shocking difference. Look around the forums, and you'll see quite a bit of raving regarding the Cardas replacement cord for the 600s, which is the one I have. Among all of the various tweaks, wires and various assorted thingamabobs that I've thrown at stereo equipment over the years, the Cardas wire on the Sennheisers was the single best and most transformative decision of the lot -- so much so that I'd hesitate to throw $$ at a new headphone amp before at least giving the Cardas a go. Many a quest for more out of 600s as started and ended just there. Not cheap, but defintely less than $500.... Just a thought.