The H9 is designed for a guitar in and will handle about 9V peak to peak, which would be a VERY hot guitar. It is NOT designed to handle a high level line input – this is presumably what you are giving it.
Your first approach would be to contact the amp manufacturer and see if there is any way to get the effects loop down to a reasonable level – you are probably not the first person with this problem.
If this fails, you could try a pad or "volume box", or have a techy friend make one with two resistors. Hopefully one of our users can recommend one.
