I've only had this for a short time but I love it so far.  The xfi soundcard on my computer didn't have enough juice to power my HD650s so I got this to use as an amp and DAC.  I'm using the optical out from my soundcard instead of USB because the USB on this DAC does not support 24-bit/96Khz (it is resampled in the DAC).  This wouldn't be an issue except that I would like to hook my xbox 360 up using the optical connection, now I have to use RCA.  The tube didn't change the "warmth" of the sound as much as I expected or would have liked, but...