What kind of amp are you using the H9 in the FX loop of?  Some amps have some crosstalk in their FX loops, which can lead to feedback.  Does your amp have a wet/dry control on the FX Loop?  I'm wondering if there's a parallel dry path or if all of the signal is going through the H9.  Ideally, you'll either want all of the signal to go through the H9 or you'll want to use kill dry on the H9 to only have the H9 output the fully wet signal…