What I posted over at HF...
Here are some very, very brief thoughts. My time with the P1u was limited and I selfishly spent more time listening than detailed comparing.
I was a fan of the 'powerful reserves, but polite' Luxman P1 presentation and the P1u continues this. They are very, very close in sound (spent some time comparing Al's P1) and I'm afraid I don't currently own a dynamic headphone amp that competes closely with either.
Much of the differences may come down to balanced operation, and their prices ranges certainly differ greatly, but using the Tony Bennett and Bill Evans XRCD (ripped and Airport Express to Parasound 1600 HD) the P1u has a larger and seemingly more accurate soundstage than my Headamp GS1 with both HD580 and K701. This was further confirmed by the more problematic, though best I've ever heard them, CD3000. A better extension in both ends (including lower presence), a more organic (though may or may not be accurate) decay were more obviously also added to the mix there (then confirm back with other cans). Overall though, the most immediately recognized difference, and all-emcompassing trait, was a more life-like atmostphere with all three cans.
Shifting over to tubes and Ennio Morricone's*The Best of 50 Years of Music (previous B&W Society of Sound club and future CD release), the Singlepower Extreme Platinum (with RCA 6SN7/IBM 5998s mostly) immediately falls behind the P1u in instrument separation and resolution (clearly can pick out more separate strings) and overall signiture balance. The Exteme seemed a bit peaky and plain uncontrolled in comparison. And again the soudstage grows.
On the discussed blackness levels, I don't have the cleanest electrical system, but I only heard slight noise when maxing the volume, and once disconnecting my source, heard nothing all the way up.
I know these trait differences are rather simplistic, but from my limited time comparing, the P1u clearly stood out above the others. More importantly it eliminated the issues I occasionally have with either, which arguably are their greatest strengths. If you feel the GS1 is a bit neutral/flat, the P1u is will bring life to the music with some signiture tweaks, if you feel the energy of the Singlepowers can often step on the music, the P1u will tame that down. While doing both it will dramatically create a more belieable soundstage. Course this is at minimum triple the cost and sacrificing a little functionality.
I've been pretty focused on Stax and K1000s the last few years, however if I was looking for a solution for conventional dymanic cans, this is where I'd start. I'd easily recommend the same for tube or solid state fans. It's probably my favorite headphone amp.