Thanks! I installed linux-image-5.9.0-0.bpo.5-amd64 and instantly reproduced the issue.
For reference: the amplifier is a Denon AVR-X500. I just purchased a Denon AVR-X1000 as replacement, I will confirm whether the issue is fixed as soon as I receive it. At this point I'm 99% sure it's the amplifier though.
