Audio PC

Low level noise through headphones can be a really irritating problem with some setups.

I’m not too sure what causes it but I don’t think it can be the proximity of the various parts.

I’ve used a tiny Lenovo PC hardly bigger than a paperback that has no interference whatsoever through headphones.

Same for many tablets and laptops.

Perhaps it’s a question of inadequate shielding? Or even a particular motherboard issue?

Although my first PC had a specialist soundcard I don’t think it sounded any better than the later ones with integrated sound chips.

I also prefer to use the minimum software (W7 driver) and stick to WMP and VLC - no enhancements / EQ.

The big issue seems to be the speakers and headphones you wish to use. It's far from easy to find a pair with a reasonably flat frequency response that you could trust for monitoring or comparison purposes.

I plugged my computer into outlet on different phase (I use WiFi).  Digital transmission may produce timing jitter that will translate to added noise, proportional to signal and present only when music is present, causing loss of clarity (during gaps it is completely silent).  Should your PC be built for this purpose - it depends.  If you use WiFi then computer doesn't matter (I use it for other tasks while playing music).  Same goes for Ethernet or USB.  Slower computer might be even better polluting less (lower speed, lower power).  With S/Pdif it might be better to dedicate computer to this task only.  You will have to experiment, but people reported better results with faster dedicated computer, free of other tasks.