PC’s are fing noisy, are you using a optical cable from the PC? This will be the best for electrical isolation, after that you can connect anything and it should be noise free all the way down the line. I have used a Optical cable from PC to a Monarchy DIP then out via digital XLR to a external DAC then to a active preamp via rca's then rca's out to a Headphone amp which drives phones or a Bose 2.0 computer audio system.
Matt
Matt