Help: Noisy Tubes

I have a Quicksilver Line Preamp with Sovtek 6DJ8 tubes. It sounded great for about 4 years when it started to sound quite thin. I replaced them with new ones from Quicksilver. Both tubes sounded great for a month then became very, very noisy (crackle, pop, etc). Quicksilver replaced one of them which also only sounded good for a month. Then I got some super-low-noise Sovtek 6DJ8s from RAM in Santa Barbara (they test and sort tubes). Same effect. Sounded great for 2 months, but now they are getting noisy too. AARGHHGHH! What the heck is going on?