Interestingly enough, going with NOS power tubes is almost like having mods done on a unit. The cost for a quartet of NOS Mullards can be a 1/3 to a 1/2 of the cost of the amp and it is debatable if the effect on the sound is worth that much. Power tubes do not last a lifetime ... preamp tubes maybe, but I have had NOS preamp tubes crap out after a few years.
Rich
Rich