I think the problem is with the tubes, but not because the tube is going bad, but because it's a low quality brand.
When I got my tube amp, I've experienced similar problem, but I could also hear the modulation that appeared at approx 1 cicle per second. I replaced a pair of GE 6k7 tubes with a pair of RCA 6k7g and modulation stoped.
Try a different brand for a pair of 12au7.
When I got my tube amp, I've experienced similar problem, but I could also hear the modulation that appeared at approx 1 cicle per second. I replaced a pair of GE 6k7 tubes with a pair of RCA 6k7g and modulation stoped.
Try a different brand for a pair of 12au7.