Some info on the burn-in for 300B tubes:( http://www.stereophile.com/features/229/index6.html ) You might read the first paragraph and footnotes on page three also. I haven't found another tube that takes as long to burn-in, but most change character(to some degree) within the first 100 hrs.
Time Needed to Break In Tubes Properly Breakin
I have had opportunity to listen to well used and brand new tubes of same brand and type and have found the brand new ones to be more extended and sligthly lean in comparison to a well used tube (5% or more of expected life used up).
It seems that as they break in, the bass and midrange increase and the highs become relatively slightly less extended and the sound richens and mellows and voices become more textured and real sounding. It appears it can actually takes close to 500 hours for tubes to truely break in. I have also noticed that some tubes that are very well used and barely above good to have lost much of their voice and are a little bit too soft sounding. The highs and upper mids loose that etch that make brass for instance sound good.
So if you have some new tubes which don't seem quit rich or naturally sounding enough but you like the basic sound after a 100 hour break in, I recommend that you give them another 400 hours of break in time before discarding them.
It seems that as they break in, the bass and midrange increase and the highs become relatively slightly less extended and the sound richens and mellows and voices become more textured and real sounding. It appears it can actually takes close to 500 hours for tubes to truely break in. I have also noticed that some tubes that are very well used and barely above good to have lost much of their voice and are a little bit too soft sounding. The highs and upper mids loose that etch that make brass for instance sound good.
So if you have some new tubes which don't seem quit rich or naturally sounding enough but you like the basic sound after a 100 hour break in, I recommend that you give them another 400 hours of break in time before discarding them.