Finally took Matt's advice and bought a media server to replace the computer. Well holy s**t ! Wish I had listened to him earlier, what a difference.
The sound is so much clearer, richer and although I wasn't aware of noise before clearly there was some because now sounds emerge from this dead silent back ground.
I used to snicker at statements like that, but I am now a believer. Being on the low-end of things, I took a different approach - but the end result was similar: removing the PC/Mac from the playback chain yielded real results, as you describe. After years of screwing around, I am finished with computer audio for playback (obviously, not for management).
For the curious... I use a Denon DNP-720AE (which I will replace, at some point). I removed the stock feet and replaced with isolator blocks and covered the top with a sorbathane sheet. Then, I connect it to my DAC with a short, quartz fiber TOSLINK cable (I like optical for isolation; I tried different cables and I've found that going high-end matters to prevent ALL drop-outs).
I use an Apple Airport Express, placed way from everything, with the single ethernet connection into the Denon player. This isolates network activity into the Denon from everything but it's own traffic. I use a CAT-7 cable for its shielding.
Finally, I run a Mac Mini, directly situated with my router, with fast outboard storage for music files. I use MinimServer for DLNA/uPnP streaming and its awesome organization, metadata handling and presentation capabilities.
The critical step: I use MinimStreamer for transcoding all audio files into 24-bit WAV. I started doing this because the Denon presented hissing sounds with some FLAC files, so I tried transcoding on-the-fly to WAV to resolve that issue; I was then surprised to hear an audible improvement (there's an interesting discussion on the MinimServer forum on this topic). I attribute the SQ improvement to the fact that WAV/24 is the least burdensome data type for the Denon player to handle; all it has to do is strip off the WAV header and feed the PCM data to the DAC.