Since about 2 months now I am using a rather simple PC and could not be more than happy with it.
http://www.atlastsolutions.com/ultimate-fanless-mini-itx-pc-core-i7-haswell-8gb-120gb-ssd-asus-q87t/Used only for XX
I should compare it with a so called optimal PC one day but compared to my previous PC there has not much changed to the sound.
At first a bit less alive and dullish but after a week on continuous power it has even improved the previous situation. Still on Switching Power but that will be changed soon so no complaints...
Using Remote Desktop (other PC, tablet or phone), Galleries on the local SSD and the music on an external server connected with wired LAN and all is very responsive.
Decoupled with bearings and a heavy weight on top to eliminate vibrations. Ultra silent, reboots (when needed) within 10 seconds (!) and small size...
Bert