6SL7s made in Russian often have leakage problems (this is a test on a tube tester) which can introduce hum. We've had such troubles with Russian-made 6SN7s (which are similar but less gain) that we've had to recommend on our website to simply not use them.
OTOH I've yet to see a Russian power tube do that. So yes, you can use other tubes and I recommend it. I would make sure though that the new 6SL7 you use is tested for no leakage on a good tester.
OTOH I've yet to see a Russian power tube do that. So yes, you can use other tubes and I recommend it. I would make sure though that the new 6SL7 you use is tested for no leakage on a good tester.