Sennheiser HD600

..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.

I neglected to mention that I do have the upgraded Cardas cables.

Well, in that case I am willing to go out an a limb and guess that upgrading to Cardas cables is almost certainly not what you are looking for. I use an older Headroom home headphone amp that I like, but must admit that I haven't compared it to anything else and am not familiar with what you're using. So, apologies, no use when it comes to your actual question. Best of luck.

If you find the HD600 thin, then barring upstream issues (your current amp is probably good enough to drive the HD600 reasonably), there's a good chance you're in the camp that greatly prefers the HD650. I'm in that camp. They have more & tighter bass, more weight to the presentation, lass treble energy, and are overall smoother with less grain. I find that despite the "darker" signature they're actually MORE resolving than the HD600.

On cables - with the HD650, I found the Zu Mobius cable best - better dynamics, better bass, and returns all the "airiness" that I thought was exclusive to the HD600. With the HD600, the Cardas was a good match - the Cardas does smooth things out a bit, but still I prefer the HD650 w/ stock cord over an HD600 w/ Cardas.

The HD600/HD650 headphones, more so than most even many higher priced headphones, respond to source, and especially to a quality vinyl setup.

On amps - well there's a whole word out there. Best I've heard was a headamp Gilmore Dynamic Reference for SS, and a Singlepower SDS (best sounding amp I've heard, though Singlepower & its owner went REAL bad those last few years) and Eddie-Current Zana Deux for tubes. There's new options I haven't tried (Beta 22, new Eddie-Currents, TTVJ Millet tubes/transformers) that should be spectacular.