Hello Sonicdeviant,
Seems like you just need to configure the operating levels of the H90 to match that of your signal flow. Usually, the front end of an amp functions at instrument level while the FX loop runs at line level. Go into the System menu > I/O > In 1/2 & Out 1/2 should be set to Inst, while In 3/In4 & Out3/Out4 should be set to Line. Here’s a video explaining the process: https://youtu.be/5y14LwN9oAk?feature=shared&t=2091
