Have you tried bypassing the effects loop? If you did a search for GK and issues, that comes up a few times. Also you local amp tech could probably tell you whats wrong with it as it's a bit hard to troubleshoot your amp over a forum.
Yea had switched back to running bass directly through Input jack in the front left hand corner of the amp, as i was not using pedal set onstage as my guitarists were bludgeing of my effects pedal leads (bloody 15 year olds! ). Could well have been problem. Also effects leads brand new, so wouldn't have any problems that i know off.
In music store under repair anyhow, should take three weeks, which kinda sucks,but be good to have it up and running again.

