This problem has recently reared its ugly head in my setup. But only through the monitors which run through a cheap Laney head. And only since I replaced the original Dell power supply with a cheap replacement item (not Dell) off eBay. Isn't there a device that goes on the plug end to kill the hum? I know you can run the signal from the laptop through a DI box, use the ground lift and then into the desk, but isn't that masking rather curing the hum?
No that is curing the hum. And lifting the AC ground is a BAD IDEA on any piece of equipment. Isolating the audio ground is both safer and the correct way to solve this if it is indeed a ground loop.