Hi, thanks for the responses. Good to be back, when I was last on the site (before the crash) there had been a project for everyone to record some clips to be patched into one big song - just as that was underway, the site was lost - so disappointing, as it was an incredible treasure-trove of info. Glad to see some of the ol' regulars still here all these years later!
RNolan - yes - SS voice is fine, you described issue perfectly - patches with clean tube voice selected in the patch are now routed to dist tubes voice and vice versa.
MarshallJMP - yes, I had originally pulled the tubeboard, replaced all the filter caps etc. When I wasn't able to get any sound out of it (years ago) I packed it up, only finally got back to it the other day - seemed to me that one of the wires that ran from the tube-board to the mainboard wasn't soldered correctly, so I removed the old solder, redid it, and presto - had sound - but the reversal issue described above.
I haven't had it apart again to play around to try to fix it, I thought someone might have an idea of exactly what it could be before I start playing around and potentially cause myself more trouble