Hi Folks...
I'm from Israel, owning a ARC-SP-9/2 + a Dynaco Stereo 70 also series 2.
Thanks to a friend of mine that works for the Army, here..., I obtain from him a lot of "O" rings, widely used for many porpouses in the army, and loaded my Pre + amp with a lot of these "O" rings.
IF...?, I really hear a Improvement....Maybe I'm dreaming!
BUT... make me feel much better!
In those 2 Amperex white PQ 6922 that I installed in my SP-9/2 they obtain 3 "O" rings each one of the tubes.
My Dynaco the two drivers 6GH8A also 3 "O" rings each tube,
for the 4 National branded East Germany Siemens, as was received 16 "O" rings in total > 4 "O" rings each tube.
Of course...as here in Israel it's an a very hot zone..., my Dynaco always work with NO grille,,, the SP-9/2 With!
Good listening...
ALEX