If you hear identical hiss in both channels, it's more likely capacitors on the amplifier side power supply. Checking/replacing preamp caps is also encouraged.
I would also replace/upgrade coupling caps that believe it or not also can go bad.
If the hiss is only to one channel, or heard substantially higher one channel vs. the other, than you can check/swap/replace tubes between both channels to ID the bad tube.
I would also replace/upgrade coupling caps that believe it or not also can go bad.
If the hiss is only to one channel, or heard substantially higher one channel vs. the other, than you can check/swap/replace tubes between both channels to ID the bad tube.