[Andrew_WOT]

Originally Posted by Penchum

Check my post here: Review of Little Dot MKV Dual Mono Solid State Headphone Amplifier It may be over simplistic, but it does get results. I have received positive feedback from owners on all of the types mentioned.

Afraid that wouldn't help with the computer source induced noise. If amplifier and PSU are in one case, USB or coax connection from computer transfers all internal computer noises using this connection as a conductor closing the ground loop. Out of everything I have tried 100% elimination is guaranteed by one of the following:
1) running laptop source off the battery
2) using xitel ground loop isolators (other brands will do to I guess)
3) adding 3 to 2 prong adapters to power cords with separate ground pin.
4) switching to optical toslink or using wi-fi as a transport.

EDIT: Actually that Ebtech Hum X | Sweetwater.com sounds like a thing to try, any idea what's inside.