I must be an outlier user of the RP-7 (original) because I tried many expensive NOS 12AU7 tubes and it simply never responded well. The stock JJ tubes always sounded best.
Unfortunately having also owned the JC5 amp I am guessing you have a very dark overall sound with that system because the RP-7 and JC-5 are already on the super smooth side of the fence.
I have also heard that (but never owned) the new Genelax tubes are very good. FYI bottom line you may regret spending a lot for NOS tubes and I do not see a need to. New tubes have come a long way.
Assuming you want to at least try to improve detail give the Yugoslavian E.I. tubes a shot- not to expensive either.

