This seems like an obvious answer to my question. BUT! For what reason have I only heard this effect for the last few months, although I have owned the amplifier for more than a year?So an impedance mismatch?
If it were only an impedance mismatch, then I would have heard this sound feature immediately (from the moment I purchased the amplifier or from the moment the burn-in period ended).
It looks like a combination of several factors at once.
As soon as the opportunity arises, I will try to send the amplifier for diagnostics. Perhaps this will shed light on the reasons for this effect.