[Diabolical Artificer]

"output from the right is low level and gets very distorted when the preamp volume is raised a little. There is also a large splat when powered down." Sounds like it's oscillating, do you have a scope? Or possibly DC on the output. When testing best to have the amp connected to a dummy load or test speaker, IE one your not bothered about loosing.

The standard procedure when fixing stuff is to do a thorough visual inspection of all components and for power amps check for shorts, EG check transistors before you apply power then slowly power it up using a variac and or lamp limiter, then check voltages. there's a lot more to it than this though.

Your amp as David says is DC coupled, these can be a nightmare to fault find and fix and to be honest not a job for the inexperienced, I'm worried you'll get in deep doo doo with this amp and potentially strain your friendship with your mate. If it were me I'd think carefully before offering to fix a friend's expensive amplifier if I wasn't too sure what I was doing.

That said it's possible to fix such a beast with little knowledge and plenty of guidance; the Quad 405 has a good deal of info written about it.

Good luck, Andy.