I have used the KEF LS50 on a desk and it was not ideal. I even put them on isoACCOUSTIC stands which improved the sound because it raised the speakers from the the desk.
Today if I was to put something on my desk again I would try the powered KEF LS50W Meta. These have DSP for desktop situations. It is also supposed to be better than the old LS50,which I still love. The other KEF option I would consider is the LSX (I think) that is also a powered speaker but smaller than the LS50W.
If you have the space consider putting the speakers behind you. It works rather well. Take a look at my home office photo on my Virtual Systems page. I have 6 monitors connected to 1 computer and still have perfect sound.
As an FYI - I moved my KEF LS50 to another room and bought a Benchmark AHB2 amp to drive it. I love this combo.
I have found that an external DAC, when the file source is internal or external hard drives, is my preferred option. I use a USB powered hub to avoid noise from computer electronics. Currently waiting on a Furman power strip to quiet it further.
Anyway I have
JRiver Media Center 27> HRT Streamer HD USB DAC> NHT PC Passive Volume Control> KRK Rokit 8 sub>ADAM Audio T7V monitors.
Waiting on a Luminous Audio Axiom II preamp to be able to use the HRT Streamers (and KRK and ADAMS) balanced inputs and outputs.
I decouple the monitors with Auralex foam.
Very happy with this setup, hoping the tweaks of the Axiom and power conditioning clean things up even more.
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 o 192.168.0.1 r 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.