It's a ground loop issue for sure. It sounds like you may have whittled down most all other options. One thing I try to do if I get that issue is to plug the computer and the speakers into the same outlet utilizing an adapter that has three outlets form one plug. It eliminates the ground path potential at least from the power source point. If it still buzzes, it is likely one of the other units power supply just doesn't work well with others.
The simple fix is to get a stereo DI box that has the needed connections for you. Most DI's have 1/4" inputs and XLR outputs. This should be fine for your needs. Simply utilize a 1/8" stereo to 1/4" dual mono cable and connect to the DI, and then go from the DI into the speakers via XLR. Then flip the respective ground lift switches on the DI until the noise goes away.