I once tried a NOS Russian 6N1P from 1965. Loved it at first because it increased gain by close to 4 db and had a real warm bottom end. But then it got to be too much on the gig, so I put back the stock Sovtek, as it's a more universal and easy-to-work-with sound with the Sovtek. Peter Montessi once suggested trying the Svetlana, but I'm happy with the stocker so I never did.
Ampeg Portaflex Club #1