If NOS pre or power tubes became unavailable altogether, what new tubes would you buy?

For what it's worth, the CV181 is NOT electrically equivalent to a 6SN7.  If you replace a 6SN7 with a CV181, it is bound to sound different, because the CV181 is operating at a different part of its performance envelope.  But people do it.  Have fun.

Raul, I think one reason that SS has taken a back seat to tubes at the higher end of the audiophile spectrum is that most of the SS gear to which we are exposed is built to a (low) price point, and if you look inside, you see crappy capacitors, bad layout, fail-safe circuit designs, for example using over-size electrolytic coupling capacitors, that assure mediocrity.  There are only a few SS products built purely for max performance with cost no object.  Perhaps you should start a thread to discuss this phenomenon and name some of the best SS products currently and previously available.

The very best 12AU7s I have heard in my system were supposedly selected 12AU7s that were made in China.  I bought them from Billington in England, and they were "Billington Gold" class.  The only drawback was a relatively short lifespan compared to any of the other 12AU7s I tried, all of which were NOS.  The best NOS by far were Amperex BugleBoys, real ones, not fake ones.  I bought them in the 70s, so I know they are the real thing.  (No one had started faking NOS tubes back then.)  Once I realized that 12AU7s per se are not the best sounding tube, I stopped caring about which brand sounds better and converted all my gear either to use 6SN7s or 6CG7/6FQ7s.  Those two have nearly identical characteristics to a 12AU7 and can be used in any circuit that uses 12AU7s, to great advantage.  But you have to do some re-wiring, in both cases. Well worth the effort.

I agree with lowrider, 6SN7s are hellishly microphonic.  Best I've tried are either KenRad 6SN7GT with or without the silvery glass coating or RCA 6SN7GT with the gray glass coating.  But you've gotta damp them somehow.  Wonderful tubes, nevertheless.  Another one to think about are 12SX7s, which require 12V filament supply and have the characteristics of a GT type (as opposed to GTA or GTB types, which can take a higher plate voltage). 12SN7s may be a bargain, too.