So about a month ago I purchased 2 NIB/NOS Sylvania 6SN7WGT driver/power tubes for my WA6, which I really, really enjoy the sound of. Last week I noticed a feedback/buzzing sound (in between songs) coming from the left driver of my RS1i's, so I pulled the tube out and re-seated it back in the socket and everything was fine for a few days, until I heard it again. So upon hearing it again I switched the tube from the left socket to the right socket, and everything was fine for a week, until tonight when I started hearing the same noise coming from my right driver. I can not hear this noise when the music is playing, and also this happens after about an hour or so after powering up, and (with no music playing) increasing/decreasing the volume has no effect on the volume of the feedback/buzzing noise, it is at a consistent low to moderate level. Being I swapped the tubes and it effected each channel tells me that the tube itself is the issue, not the amp.
Is this something that may be a little common with tubes, should I be concerned about this?
Does this feedback/buzzing sound mean that this tube is no good, or on its way out, or is there something I can do to stop this from happening? I would appreciate any comments on the issue, I'm pretty new to the tube amps, and have a long way to go as far as knowledge, thanks.