Gonk,

I didn't ask that question, but, based on his response, the answer appears to be "yes." He understood precisely what I'm after and knows the amp, its his job; I don't think there's any mistake (reinforced by his idea of using a unity gain method). I could call again, to be sure, but my sense of the conversation was that he knew exactly what he was saying (understood my concern, immediately).

I think your idea of "global behaviour" is correct. While unrelated, directly, it's built so that the five channels share two torroidal transformers.

More and more it seems like it's either one way or the other, unfortunately.

On the other hand, the original owner of the company said it would be possible with the use of an additional switch, inputs and rewiring. What it would cost me and whether it's worth it, that's another question.