I've been (attempting) a repair on one of these - a real old British cottage industry device from the '80s or so, built like a brick outhouse. Couldn't find a schematic anywhere - only an owner's manual.
The power amp layout looked sort of familiar, so I looked through my data of such stuff, and it's remarkably similar to the much missed Maplin 150w MOSFET kit of the '90s. Wonder if they had the same designer?