Amazon.com: EB Tech Hum X Voltage Hum Filter: Musical Instruments
I'm really hoping I plug it in and it will magically get rid of my issue. For some reason, I feel doubt, but we will see. I'll let you know how it goes.
There is two possible solutions for your problem.
First is cheapest, you should provide ground to your headphone amp. It would be best that it's power supply provides GND reference.
This is cheaper solution but not 100% guarantee that hum will disappear.
Second solution would be the best. You need two audio transformers for galvanic isolation of your headphone amp from audio sources which have GND issues.
PC's are a nightmare when it comes to sound. I don't like that technology. They are not build with Hi-Fi in mind.
Hope it helped.