I'm afraid I don't really understand your description, but:
1) Mr baron58 is correct on the H9 levels – some effect loops are too hot and will overdrive it. The H9 is designed to be used at guitar level.
2) If you (as you imply) are using a parallel effect loop, you should usually have KILLDRY on to avoid the sort of feedback you describe. But, this will cause problems if you have more that one pedal in the loop. No solution to this – it is the nature of parallel loops.