I think the problem could be due to one side having stronger tubes than the other side.

The Torii MK4 is actually 2 mono amps, one for the Left and right channel.

Do u have a tube tester?

If not, there is still a way to identify the culprit.

If you hold all other tubes constant, just swap the Left and right channel tubes with one another.

e.g. Preamp tubes
Output tubes
Voltage regulation tubes
Rectifier

eventually, you should be able to identify the set of tubes responsible for the channel imbalance.