Ground loop noise goes away when heater comes on
Quick way to improve ground loops is to connect a decent thick earth cable between all the audio devices.

If you have a typical 1 mA leakage current, carried by a long cheap audio cable ground, easily a few Ohms, it will generate a few mV, which gets added to the signal, and will be audible.

With a fat ground wire, the voltage drop between the two chassis will be a hundred times smaller.

It should be quick to try with a piece of household mains wire.

This brute force method is recommended (by me) even if you do the other things, adding transformers, etc.