Please pardon my ignorance, But what exactly are NOS tubes ? Are they really old tubes, have they been restored? Are they worth the $$. Do they last ? Again, sorry for the dumbness, but just really starting to pay attention to my tubes.
Maz JRnr head Z-best 1x12 cab w/ Gold 2x10 cab All in RED Suhr Classic T Nocaster R8 Bunch of pedals
NOS can make a huge difference when used in the first gain stage ... V1 in most amps.
In the past a 12AX7 was supposed to have an actual gain of 100 at 250 plate volts, 2 volt bias and 62.5K of plate resistance. Most NOS types were 90+.
Today's tubes run between 75-80 on average and 10% difference in the first gain stage will turn a great lively amp into a ho-hum amp faster than a dropped tube will hit the floor.
In my clients amps with production tubes each tube is tested and I may have to go through 20+ tubes to find one with a true gain of over 85.
As a side note, you cannot measure or equate gain to transconductance. Just because a tube reads high on a TC test does not mean it has proper gain. High TC with low plate resistance means no gain. The old long smooth plate Ei tubes had very low TC and very low current output but with plate resistance in excess of 80K at times they could have an actual gain of 100-110 commonly. This is one reason they were prone to physical microphnics ... proper gain but sloppy internal construction. When the constriction is proper they are spectacular tubes and they were done on the same tooling that was used to produce the famed Telefunken long smooth plate.
I know it's a little late to chime in, but, if you want to experiment with NOS tubes get a rectifier tube first. You might not notice a huge tone change but they will last a very, very long time. It beats the heck out of your amp going dead and blowing fuses mid set. They are a little pricey but it is worth it, trust me. Ask Myles who posted before me and he'll agree. Then after that, like Myles said, change your V1 for whatever NOS tube meets your specs (ie. less gain, more gain, smoother, etc.). It'll give your amp a whole new voice and gain structure. Then later on start on the other slots. Just my .02....