send an email to Charley or Jorg over at Bogner, they have been exceptionally helpful to me over the years, also you can try replacing a couple of pre tubes, each tube serves two functions so maybe one of the side of one the tubes is bad.
Channel 1: V1, (V4 only used if effects loop is on), V5, V6
Channel 2: V1, V2, V3, (V4 only used if effects loop is on), V5
Channel 3: V1, V2, V3, (V4 only used if effects loop is on), V5
Channel 2 and 3 PLEXI MODE: V1, V3, (V4 only used if effects loop is on), V5
