Volume failure on mk5

[Pete1340]

Mk5 90, worked fine then next time I used all the volume gone. Can hear each channel but no volume on each. Plugged guitar into fx return and it sounded ok so assume power stage working ok. Tone controls work on each channel, gain seems to be operative but the volume on each channel doesn’t effect the volume of output. No reverb on chan 1 and 2 but on chan 3 it surges up and then levels off. I believe the amp had a service and new reverb tank just before I bought (2weeks ago). About to try swapping valves as I have some out of a triaxis (which is another boogie product bought 1year ago and ld1 Red failed after a week) really not happy now with boogie amps. 2 bought 2 failed. Any advice welcome.