This may sound like a plug, and I suppose it is in a way, but I had to let others know how happy I am with the amp Tangent built for me just a few weeks ago. I bought a pair of AKG 501s a few months ago and was suprisingly happy with their sound playing CDs on my Pioneer Elite DVD player and runing it into a Pioneer Elite receiver.
But I knew I had to get a dedicated amp to hear them at their best and use my dedicated stereo equipment. So I ordered a META42 with crossfeed.
As I hoped, things just got better. The bass tightened up and seems to extend further, the highs lost some of their brightness, and the midrange became more like my tube integrated than I could have hoped for. I have no frame of reference as I've not heard any other dedicated headphone amps, but I do heartily recommend Tangent's stuff at this point. And as for the AKGs, I've read post after post about their not having any bass, but I must disagree. They may not plumb the deepest depths, but their bass is clean, articulate, and real to my ears. I listen mostly to jazz, so perhaps my opinion will change when I start throwing some rock at them, but I doubt it.
In case anyone is interested, I'm now running my Arcam Alpha 8 directly into the META42, when it's not connected to the tubes of course.