it's possible that the NOS have slightly different specs and aren't a good match for the SF circuit. my CAT pre is picky that way - it also likes the Sovteks better than some NOS I've tried.
Preamp Eats tubes..
I own a sonic Frontiers line-3 preamp.The unit eats up NOS tubes.I tried Brimare and matsushita's both within 2 or 3 months the tubes started going microphonic on me.I put in the origanal Sovtek tubes and they work fine.Why does this happen with older style tubes?Are the sovtek tubes built for a tougher demand?