Actually we did a long and careful comparison of two identical Fet Valve CF preamps, one with our stock JJ tubes and the other with the actual NOS tubes that TAS used for the review using our ABX box, a Fet Valve 600R power amp, Fet Valve DAC, Vision adjustable phono preamp. and a set of Philharmonic Three speakers.
We could not reliably pick out any useful differences between the two preamps. Any improvements that the NOS tubes are claimed to provide were so small that we could not pick them out. Our choices came out random. We even swapped the tubes between the two units to make sure there was no bias between one of the identical preamps or the other. "Goodness" did not follow the tubes, it remained equal with both.
Would I spend an extra $200 for the NOS tubes? No. But if that extra expense makes you feel good, be our guest.
The review is just as meaningful with our standard JJ 12AT7 and 12AU7 tubes as with the NOS tubes.
I am disappointed that the review did not use our stock tubes but the Fet Valve CF remains an outstanding preamp all the same.
Frank Van Alstine