Sounds like a ground loop. If the amp's power cord is plugged into a socket that has a ground connection and the H9's power cord (or any other gear for that matter) is plugged in somewhere else, there could be a ground differential. One possibility is that the effects loop is grounding the H9 at the signal ground and that ground is different than the power ground. Have you tried plugging the H9's power into the same place that the amp's power is connected?
