Rockmaster preamp - low output

[johnylondon]

I'm afraid my somewhat unreliable Rockmaster is at it again. This time the symptoms are low output across all channels (and rather thin sounding too). I have cleaned rear jack connections (internal switching ones) plus using patch leads, with/without footswitch. Tried swapping out the valves one at a time with a known good one (curiously another super 7 that came out my ADA!) but to no avail. While I was in there I replaced the two biggest PSU caps as they were bulging massively and prior to this a fuse had stopped working. (The fuse that is on its own, the other two are ok). I say "stopped working" because visually it was fine, I think it must have just failed internally from old age rather than "blowing" as such as the thin strand looked alright.
Obviously, checked the board for dry joints etc, and tubes reseated several times. I'm out of ideas unless it is those pesky relays AGAIN. I do have the circuit diagram but it is very difficult to read - you have to zoom in a ways and once you do, it's easy to lose track of where you are.
Anyone have any ideas?

[j0nsnell]

Possibly a low emision valve or low value coupling capacitor.
I would trace the signal as it goes through the pre amp with my oscilloscope to see where the signal gets bad.