Rob Watts will happily tell you that Dave’s USB input isolation is not perfect… there is still coupling capacitance. So there is a mechanism for different USB sources to potentially sound different, even with Dave… Your observation doesn’t surprise me at all and I observed the same when I had Dave…
TOSlink, on the other hand, provides perfect isolation from mains RF / groundloops / leakage current loops. This is why Rob himself will tell you TOSlink is the input to beat with his DACs… incoming jitter is not a concern with his DACs…
On the Dave thread on Head-Fi:
He’s talking from the technical perspective (as the designer…). If someone prefers other inputs and other sources, it’s possible they are preferring slightly increased RF / noise.
Nothing at all wrong with that of course but if the designer says the TOSlink input is technically the best, it’s probably correct for that particular DAC.
If someone thinks Rob has it wrong with Dave, don’t argue with me - go and tell him over on Head-Fi 