The final part of this story: An authorized peavey workshop has just opened up 5minutes down the road from me & after two visits the head is fixed. It was a dodgy pre-amp tube. It was the V5 position which interestly i found out is the only tube that will still "sound" when the gains are turned to zero. So if you get random crackle & noise on your 6505 with the vol at zero it is likely to be the V5 position tube.
