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?
Sounds like it's time to send the unit for a check up. Could
be that some of the operating voltages are out-of-spec on
the high side (failed cap/resistor?) and prematurely blowing
out the tubes. Although I recently retubed my LS5mkII with
new Sovtek 6922s, the originals are still in perfect working
order after 7 years of fairly heavy use and are now serving
as my emergency back-up set. Preamp tubes should last quite
a long time if the unit isn't left powered-on 24/7.
i also believe that problem is beyond tubes and you should check the rest of circuit for bad caps mainly.

Specifically, have the bypass caps and cathode resistors checked. If the unit has had it's electrolytic main filters replaced---probably a good idea to replace those 'lytics on the gain stage and input tubes.
Thanks for the advice. Sounds like it needs a tune up. Will send back to the manufacturer...