Have done some more testing....lifted the legs of the output transistors Q1 - Q4.
Q1 and Q3 appear to be good - 10 ohms between base collector and base emitter one way, open the opposite way, and open both ways collector emitter.
Q2 and Q4 both appear bad however. Q2 reads 10 ohms base collector and base emitter one way, and 38 ohms going in the other direction. Collector emitter is less than 1 ohm both ways. Q4 is less than 1 ohm on all combinations !.
Also tested Q5 and Q6 and these appear to be good, however Q6 appeared to have a dry solder join at it's base. Both Q5 and Q6 are C4137, however I haven't been able to find a pin out diagram for this. Only datasheet I've found says it's used for audio temperature compensation.
Is it worth replacing Q2 and Q4 ? ($22 cost). Do these readings help explain the initial symptoms and the blowing of R308 ?. Would a dry solder on Q6 have any significance?. If Q1 and Q3 are good why did that channel have voltage on it's output ?.
Any help / suggestions would be much appreciated.