What is the best 12ax7 NOS tube ?????????

It really depends on the peice your placing them into. I have tried Telefunken and Bugle boy and didn't care for them any better than my Stock tubes.There is no such thing as the best,its all a matter of taste. I would suggest try some good used tube to see if the sound your getting is what your looking for. If you buy NOS and don't like them your out the money and they are not NOS any more to sell. Good luck

Do power tubes have much of a sonic signature? I'm curious as to what the sonic benefits are of rare and expensive NOS power tubes (EL34's in particular) costing sometimes hundreds of dollars each compared to new ones that run about 25.00 a pair on average. What are the benefits of NOS power tubes? Are NOS power tubes that much more refined sonically, do they have a longer life etc etc?

Both is true Eric, they often sound more refinded and they last much longer. (Especially the KT 88)