Tried out the effects loop over the weekend and whether I run one pedal through it, or loads, tried different cables, power supply or battery, the loop adds a high pitched whine to the sound, which makes it pretty unusable.

I'm going to re-try a load of things, again different pedals, batteries and cables, but do Marshall have any suggestions?

Also is there a way to change the level/mix of the effects loop, or is it fixed?