So get this:
My preamp only has single ended outs, and it was connected to an unbalanced amp using RCA cables. With zero ground loops. In fact I went through several amps and preamps (all unbalanced) with no ground loops.
Now I got a balanced amp and connected it to my unbalanced preamp with
regular RCA to XLR cables - no ground loop.
Then I got this transparent converter to convert the unbalanced outs to true balanced -
https://orchardaudio.com/product/rca-to-xlr-converter-module/
And now that it's balanced - I'm getting ground loops.
Everything is plugged into the same outlet/strip, and I'm using a high quality power supply for the converter (I tried several power supplies).
So what can I do?