? Tube Replacement Question ?

I always buy a new tube set when I get a new piece of equipment. Worst case, (outside of spending extra money) is I then have a spare set which is very useful, especially in diagnosing quickly if I have a tube problem or something else has gone wrong. Normally I do not buy new tubes from the manufacturer. Not only are they generally overpriced, but they are generally new production only because the company has to keep a large stock of known quality tubes on hand. The bigger the company the bigger this consideration.
I suggest you are doing the right thing posting on the internet for advice. Keep reading until you have the names of the handful of reliable suppliers of NOS and speciality tubes to Audiogon and Audio Asylum members.

One of the problems with tubes is once your start "thinking" that it might be time to change the tubes, your doomed, you might as well change them out because in the back of your mind, you will be always questioning the tube's performance, even if they are 100% good.

Something VERY important, when purchasing preamp tubes; being CERTAIN they are CERTIFIED LOW NOISE. ie: Upscale Audio stringently tests and grades their tubes, per their noise, microphony, and triode balance(VERY dependable). ie: (http://www.upscaleaudio.com/tubes/12ax7/ge-12ax7.html)

Some of the NOS Mullard, Telefunken, and other tubes seem to keep on going in some gear, 10 years plus, only a couple of years in other. I'm talking high hour daily use also. Usually with the good NOS ones, as long as they test good, their sound doesn't seem to deteriorate. If you plan on staying with tubes, I would consider buying a Mutual Conductance tube tester. All of the tube amp people I know have extra tubes on hand. The quality of some of the newer ones are getting better, but I don't think they've reached what some NOS ones can do. Others say they've have it down now, for sonics and reliability.

http://www.audiocircle.com/index.php?topic=43982.0
It's not easy getting a quiet reliable tube.