B E F O R E you do anything, find out what the tubes do in the circuit:
- do they provide gain or just buffer for cachet
- are they used in a balanced configuration or single ended
- if balanced configuration, are they actively balanced or fixed
NOS tubes can be gassy and take a while to come to life... or may never 😞
This is four NOS 5963 [similar to 12AU7] used as a driver stage in two amps. There are 24 possible combinations of these tubes. Each combination sounds different.
And read ieLogical Rolling