Ultimately, the feedback loop is tuned for stability and performance with the PA774 output transformers, and the Edcor's need to be separately tuned. I hope to do that this year - a full Edcor setup - when I mess with the Rev EFB. In the meantime, I would see if you get a greater margin of stability with the NFB pot adjusted for something in the 30K to 40K range. I'll get those picture links to you tonight. Look sharp!
Shannon