Audio Research Sp16 tube upgrade

I would like to know if anyone has a Sp16 that they have upgraded the 12ax7 in and if it makes a significant difference. What nos tube should be used and are the tube dampers necessary?
I can't help you on NOS choices-- but the use of tube dampers is strictly to taste. I'd imagine the SP16, given its base sound, benefits from dampers-- but you might prefer it otherwise. Better than the dampers provided by Audio Research are the dampers from Herbie's Audio Labs. Not only do they sound better, but they don't bond to the tube with use, so you can experiment with the dampers on and off. They're great on the tubes in my SP9 Mk III.
I have NOS Teslas in my SP16 which give greater extention and detail without whitening the sound.
These Tubes are almost carbon copies of Telefunkens and the Hong Kong Audio society couldn't split them in sound.
Please note that these are not the current production JJ Teslas.
I also have Herbie Audio Lab dampeners and believe they are a worthwhile upgrade.
Best thing to do is to do some tube rolling to find out which you prefer.
I haven't found profound differences but suptle ones.