After 48hrs of continuosly burning-in, I tested usb drivers and external power this afternoon. Vista noSP, foobar 1.0, no name printer usb cable.
My preference is for X2-ASIO (the beta driver Tom sent to me), second WASAPI very near and last DS that sounds too much bright, sometime harsh (i.e. cymbals), for my taste. ASIO confronted to WASAPI delivers a more credible soundstage, more air around instruments and voices. I didn't get any booming bass frequencies, they're correct.
All of this in my opinion, obviously.
However all the three sound worst than my reference shigaclone CD player when I run CD ripped material (pcm or flac converted).
24/96, 24/48 or 44.1/16 master is another story.
I should add that sometime I got a great level of noise with Wasapi that forced me to change the usb driver in foobar.
My great surprise was with external power. I built a simple AC filtered regulated power supply based on LM7808 that delivers stable 8Vdc, nothing extraordinary, but it should be better than usb 5Vdc. I was expecting an improvement in sound quality, maybe slight, but nothing. I can't say I heard any difference between the two.
It was quite disappointing cause I took for granted that better ps means better sound quality, but it has not been the case.
Next test will be I2S. I've no experience with I2S, I never owned digital sources with I2S output, SPDIF only, so I don't know what to expect.
EDIT: tests with coaxial SPDIF, a 5m Belden 1694a CanareRCP RCA plugs