Why don't you just resolder everything?
You should at least resolder the tube sockets and any large heavy component just because you are in there already, that's par for the course for me.
Resoldering everything will take you what, 10 minutes?
One thing you might check is that every resistor connected to pins 1 & 6 of each pre-amp socket is intact.
These are the Anode resistors for each side of each valve.
These do for whatever reason sometimes fail open circuit (sometimes a faulty pre-amp valve) and the result is an often an unusally quiet amp.
Nada, no hiss, nothing.
