Distorted sound on k4040

I have a k4040 which I built about 9 years ago. It has been working well all this while and sounded perfect. Recently, there have been distortion on the denser parts of the music especially in the midrange and when bass notes are present. This only happens on the right channel. I have swapped the valve tubes around the pre-amp tubes have been swapped from left to right as well as everything else like the speakers and pre-amps that connect to the amp. The distortion still comes from the right channel. Could it be the output transformers? I suspect it’s from one of the components on the board, maybe a resistor or a capacitor that has failed. I dont know which to test and how. Anybody have any advice? Thanks.

Try swapping the output transformer, in order to find out if it is at the cause of troubles.

Before I swap the output transformers, (Theyre huge), I would like to ask, is distortion, even at low volumes the characteristic behaviour of failed output transformers?

Thanks for your advice.

Not sure but possible.