I've had my Laney for about 2 years and all of a sudden when I turn it on, even if all volumes are set to 0 and no cable is plugged in there is a constant humming. An employee in a guitar shop told me to replace the valves, so my dad ordered two and replaced two of them but it is still the same.

Shall I get the rest of the valves and fit them in or will that not solve the problem?

Thanks in advance!

(sorry if this is in the wrong section but all the smart people hang out in this part)