The first troubleshooting step should be to swap the interconnect cables at the amplifier input end and see if the issue moves to the other side. This will show that the issue is in the pre-amp (or possibly source) and not in the amplifier. If you determine that the issue is in the pre-amp or source, you can forget about troubleshooting the amp and speakers. You can swap other cables to isolate the pre-amp or source as the cause of the issue. Once you know the problem is in a certain component, then troubleshoot that component. If it's the pre-amp, swap the tubes side to side to see if the problem moves.
Of course, turn off the power to everything when you're swapping cables and especially tubes......
Of course, turn off the power to everything when you're swapping cables and especially tubes......