VM100 Oscillation

Hi, I purchased two VM100 from Amazon. They are built into a metal cabinet.
Both do not sound good and an oscilloscope shows why - oscillation when connected to a speaker! (not when connected to a resistor).
Another thread from early 2019 mentioned it might be a ground loop when two amps are in the same cabinet, but both amps oscillate even if alone in the cabinet (the other completely removed).

Maybe they need a small cap somewhere - but where?