New production tubes get the rap for being short lived, and going microphonic. They appear to be "hit and miss".
NOS do seem to "live longer", and as far as different tone form different brands, I can say first hand that it's true.
I was lucky and had several NOS from years back, and there is a difference in tone between brands.
Subtle in most cases, eye opening in a couple. My tweed circuit Fenders loved RCA preamp tubes. For whatever reason, there was a certain "more open" sound from them in tweeds.
Power tubes generally will have a more dramatic difference between brands. Each brand has it's own "coloring" too, just like preamp tubes.
My favorite 6v6 is the Westinghouse/Marconi "coke bottle" tubes. So darn musical it ain't funny, and a rich breakup that's insanely good sounding.
RCA "Blackplates" seem to be a standard for 6l6's, and to my ears, I can see why. Very rich sounding with outstanding breakup.
New production "GE" 6l6's sound very good too. The ones that came stock in my RI Fender '57 TwinAmp sound excellent. How long they will last is still unknown.
Now I will talk "bad" about Mullards. I never put a Mullard in a Fender that sounded good to me. Not as good as an RCA/GE/Sylvania/TungSol
However, put that same tube(s)in my Vox, and Holy Moly, the Holy Grail of Vox sound appears. IMO, Mullards seem a little more nasal or midrangey, which I don't like in a Fender, but do LOVE in a Vox! Sheesh, a Brit tube that sounds better in a Brit amp
I could go on forever about tubes beacause I've toyed with them for years with different amps.
A tube is a tube, and a string is a string
No, not really.
Edit: NOS Tesla's(not JJ) el84's sound very, very good in my Vox. Don't have the jack for Mullard el84's, and the Tesla's are keeping me very happy.
Be careful on the Bay. A lot of "fakes" out there that are just "rebadged" new production stuff. Also a lot of "pulls" that are sold as NOS.
I've had great luck with www.kcanostubes.com
Mike's a straight shooter, and you'll get what he advertises.
Have some fun and keep us posted.