since tube amps are symmetrical, whenever there's something going on soundwise which is tube-related, you can do things like swap out tubes like you did or, if the funny business is in one channel, switch the left and right channel tubes one by one to isolate the problem. this is especially true for input tubes like your 6sn7s or preamp tubes. bad tubes often make nasty sounds when you tap on them. I don't think it's a good idea to get into the mind-set of 'there's something wrong with my tubes that I can't actually hear'.
assuming you're biasing your tubes, that will tell you how your power tubes are doing: if they're having trouble holding a bias, you may have a bad one. and eventually they just fail, often in very unsubtle ways with loud noises.
assuming you're biasing your tubes, that will tell you how your power tubes are doing: if they're having trouble holding a bias, you may have a bad one. and eventually they just fail, often in very unsubtle ways with loud noises.