My preamp is Conrad-Johnson's top model the ART88. It came brand new from the factory with four Phillips NOS ECC88's. I read in a HiFI+ review that you could replace the "made for TVs" NOS tubes with EH6922 Golds for a nice increase in sound quality. Okay, so ordered a set. Yes the change was immediate. Jeff, the designer at C-J said he thought the EH's might turn noisy but no harm. After a few months, there was no noise from the EH's but thought I'd put the NOS tubes back in and see how they sounded. What I discovered was a definite preference for the original NOS tubes. The sound was better; richer in the mids and just as detailed. Turns out, I was fooled by a bit of extra gain from the new production tubes. Slightly louder, of course, sounded better at first.
Guess the process is to listen and see what you prefer. The HiFi + review has different gear so came to a another conclusion.