Hi David12. To be candid here.... I have never heard your preamp in my own rig, however, I would like to point out that the typical 6922,6dj8 family of valves will traditionally last in excess of 3,000 to 4,000 hours of playing time in the CJ.Unless the tubes have become noisy or microphonic, I can't believe they would not test within spec for your preamp. NOS vrs current production tubes is a very subjective topic to be sure. One must also take into consideration the fact that almost all of the better modern tubed gear being manufactured today has been voiced with current production tubes. That is no guarantee the preamp will sound any better with good low noise NOS valves. In my experience: regardless of modern or vintage gear [preamps in particular],a strong testing low noise nos 6922 tube with tightly matched triode sections would inevitably win hands down in any given shoot out. Those that refuse to believe it are the people that  did a comparison with tired nos tubes used nos tubes with poorly matched triode sections used noisey or microphonic nos tubes  those that refuse to pay the current market value for a quiet and strong testing vintage Seimen,telefunken,Dutch Amperex[to name a few] 6922 family of tubes. Furthermore; sometimes the gain in sonics may not be justified given the ever increasing cost of good nos valves.I for one, believe the premium selected and matched Eh tubes are decent sounding in your preamp.... the russian military 6n23e's are considerably better [by a wide margin] and are always dead quiet to boot.They can be had for the same money with a little searching.I really like cellorover's anology with regards to establishing a benchmark for comparison. Do you have a tube tester? I would take bets that your stock tubes would test better than 90 percent. Perhaps, cellorover's suggestion of up grading to the mk2 version is in fact "the bigger bang for the buck". All the best in your journey.