Those switching supplies are deadly when it comes to noise. My son only has one A/C circuit in his room (not his wouldn't pass code today, but that is how the houses were wired back in the 50s). I got him a PS Audio Noise Harvester to try to clean up the feed to his stereo the best we can and you would not believe how that little thing goes crazy when he switches the computer on. Without the PC it blinks at the rate of one flash every 1-2 seconds. Flip on the PC and the flash rate goes up to about 10 per second! I've never seen a linear PC power supply,(then again I never really looked for one either), that sounds like a good idea for a music server.
When you say you are using a PC as a transport do you mean that you are playing CDs through it in real time or ripping the CDs to the hard drive and playing back those files. One of the big advantages of using a PC as a music server is that by not trying to read the optical disc in real time you have the opportunity to get better error correction and therefore a cleaner signal.If you are playing CDs in real time it does not surprise me than audiophile dedicated transport could do a better job in that regard. The transport was of course optimized for realtime playback and will (or should?) have better mechanical isolation than a $30 stock computer CD drive. However the CD drive can produce excellent results when not saddled with the obligation to perform in realtime. Optical disc are notoriously unstable in their dynamic rotation introducing all kinds of data errors that the CD playback system has to either figure out how to correct or replace with an interpolated value. I think we can all agree that the less number of times you are forced to interpolate (guess) the better your source will sound. When you are not operating in real time the CD drive will have the time to go back and re-read any bad sectors and try to get a better read on the corrupt data. With less unrecoverable data you need less interpolation and get a more accurate reproduction. So, a cheap drive can perform very well if given the time to run more complex error correction, but if it as to get it right in realtime, an audiophile designed transport will have a better (and more expensive) designed that will help lower those errors before they ssrt through better structural design.
In the case of a PC drive the unit will, for the vast majority of people spend very little time playing back music so there was no real reason for the design engineers to build in (or pay for) a level of structural integrity that ill be used only by a small percentage of owners and even within that small group, how many of them will have the system, the time , or the inclination to worry about the sound quality of their PC's sound system.
