With the fault persisting across multiple sources, and across 2 pairs of speakers, it's almost certainly the amplifier.
One very simple test is to try a different amplifier.
The last non-valved amplifiers I've bought have been:
a brand new NVA A20 power amp in a sale for £133 direct from manufacturer
a used NVA P50 SA passive pre-amp for £166 off ebay
a Urei 6400 4 channel amp with 1 channel broken for £15 - ebay
A Urei 6230 2 channel power amp with volume control for £80 - ebay
2 Urei 6290 high powered amps with volume controls for £150 each - ebay
The point being that you can buy some very nice sounding solid state amps for not a lot of money if you know what to look for, have some patience, and an open mind.
One very simple test is to try a different amplifier.
The last non-valved amplifiers I've bought have been:
a brand new NVA A20 power amp in a sale for £133 direct from manufacturer
a used NVA P50 SA passive pre-amp for £166 off ebay
a Urei 6400 4 channel amp with 1 channel broken for £15 - ebay
A Urei 6230 2 channel power amp with volume control for £80 - ebay
2 Urei 6290 high powered amps with volume controls for £150 each - ebay
The point being that you can buy some very nice sounding solid state amps for not a lot of money if you know what to look for, have some patience, and an open mind.