Maybe something like the guitar shielding or your PC monitor in proximity to the HD500 PSU? Try moving the PSU around. I'm just tossing out ideas at this point. My bro has an HD500 and a few BL PUP loaded guitars with no hum issues. I use a FW DI unit into my PC. My BL (NF series) loaded guitar is dead silent through it. It's my most quiet guitar.