Just got myself a Quad 306 (M12890-6, latest board), with one working and one dead channel (just hum).
I haven't powered it up yet, but there are a couple of burned out resistors that are obvious, and some weird looking 'residue' around a couple of the Power Transistors. Ran a few very brief checks, there are no short circuits on any of the transistors, so I'm hoping that they are OK...