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Presumably you have 4 cables going from the the H9 to and from the amp. This means that there are 4 ground connections, which may be too many.
My guess is that if you unsolder/cut one or more of the grounds going from the H9 to the amp, this may fix it.
But, to do this, you would have to be moderately technical, and I don't wish to assume that you are. Do you have any technical friends who can help ?
Otherwise, keep the cables as short as possible, and the same length. Ground loops more commonly cause hum, but they can have other effects.
Lest you think that this issue only applies to our pedals: