I have mine for two weeks now and just LOVE the sound it give my Sennheiser HD598
I only use USB power, I try but don't hear any difference in the sound quality on my HD598.
I use it together with a S.M.S.L SA-S3 T-Amp, and ProAc_Response 1SC Speakers
Talk about Micro Stereo with a BIG sound :-)
But you need a clean USB with full 5v power, (Not a USB HUB) to drive it
Fantastic sound from all of it