I,ve tried a number of system optimisation packages over the years, including some that are bundled with music players. For Windows machines I still prefer the packages offered by Black Viper which aren’t primarily aimed at audiophiles.A further advantage with the Black Viper package is you can cross reference with the manual instruction pages which give you quite detailed information on exactly what processes you’ve disabled and what they do. Trying to get accurate information about exactly what most other packages and music players do is nigh on impossible.

In my opinion, for such software to make a significant impact on sound quality it needs to filter and/or re-sample; I don't want either.

Add to this that on the very few occasions I have heard any differences they have’nt in my system been for the better.

In my experience what can make a significant improvements to the sound are hardware based, mainly concerning the supply of clean power and the separation of power rails carrying audio signals from those that don’t; this usually involves some reworking of the motherboard which isn’t at all straightforward and can be expensive if for example you wish to supply say USB using a linear regulated power supply.

I still find some of the Linux distributions easiest to use for sound reproduction, especially if you believe that background system activity has an effect, mainly because you tend not to have to worry about disabling system processes because not a lot runs as default.

I was surprised by how much of a difference a decent linear PSU made with my Squeezebox Touch (which I use as a transport), and I used the same PSU with the EMU 0404 USB ADC/DAC that I used to evaluate Fidelizer.
The PSU makes a very noticeable difference.
There's no power at all over the USB connection - the EMU 0404 USB was designed to work solely with an external PSU.