We got it today, ended up loading the shotgun a little bit. Through lifting jumpers/bus connections to the Channel 1 pre amp we isolated the problem to only that channel. Already having changed the finals we worked backwards, pulled the pre amp and swapped it, no joy. Then looked at the biasing transistor that fed the pre amp, bingo! I'm glad I bought the parts amp, has a good power supply. Hopefully it stays stable and I'm not pulling it back out soon! Thanks to all for the comments and ideas, I learned quite a bit from all of this.
