Music Man Amps

Hi long time musicman enjoyer,
finally bought a rd50112 that I thought would be a good grab and go.
It sounded like it had a buzz in the speaker but playing another amp through the speaker and it was fine.Buzz on the clean channel (and on the gain channel but i'm concentrating on clean channel). Up on the scope I see what looks like crossover distortion even at a low level. (It can get loud and proud but ugly). Looking through the many posts here on the rd50 I start to get handle on it. I notice that someone has put a 22k resistor on top of c9 the 30 volt vener diode.
raising the bias from 31.5 to about 37 volts. I imagine that this was to deal with the crossover dist(I dont know how stable this would be under drive). I removed it and can say it made no difference to the scope or the crossover dist. Tried 3 different sets of tubes in it, same problem. I am now thinking its in the circuit. Tracing signal through, it seem clean through ic1 and ic3 . Exiting ic4 is seems to have a little of the crossover dist in the wave form but this is after feedback loop. The output of the drive transistors measured at pin 8 of the 6l6's seems a smooth half wave and added together make a nice sine wave (there is very little overlap where one starts and the other leaves off but this is probably a different class of amp from what Im used to.)

I have not replaced the electro caps yet but I dont feel that this is the problem.
Its a 250 volt power trans running on 240 volt Australian power
power supplies are
560 v anode
279 v screen
+16.7 - 16.3 +7.5 -7.28
bias having removed mod 31.3
does this sound like replacing driver transistors ?
I dont really wish to flummox about replacing things without understanding.
Sorry if this is a lot of information but I thought Id do it all in one lump :)
Any helpful hints gladly accepted