[Radio Wrangler]

I used to design frequency synthesisers at HP, which was pretty much the deep end of tailoring the responses of feedback loops. Some audio amplifiers are designed fairly close to the border of stability problems and it isn't directly visible just how thin was the ice they were skating on. This goes for some well-renowned amplifiers. They were in fact working at the limits of valve technology, short of the big step to output transformer-less designs.

David