Thanks for the idea's people. I can confirm that there was nothing plugged into any inputs the PSR.

However I've been looking at the system. There are some "interesting" wiring anomalies to do with the way the induction loop system is wired up. Apperently there were "difficulties" with that unit some time ago and whoever came in to do the job may have got it working, but now the loop system only works when both the "Aux 1" and "Foldback 1" outputs of the desk are plugged in, even though the loop system should be driven solely by Aux1. If I plug the small amp into the Foldback output of the stage box, then any gain applied to the output gives feedback.

On this basis I suspect that the loop system, which is in a cupboard separate from both the stagebox and the desk, but which also contains all the amps, is cross-wired to the foldback somehow. I can mute every input and output of the desk and not alter the feedback; the only way to stop the feedback (other than turning off the amp) is to remove either the Aux1 or Foldback1 from the desk.

Hopefully tomorrow I'll get some time to check each circuit in isolation.