I have had challenges with HX Stomp’s effects loops in the past. In fact, it had to do with level mismatches. You can see people complain about it in their forums. Check out this reply I got when I contacted their support back in 2021 about it. Maybe making these adjustments might help you. Switching the effects loop to line level for H9 worked well for me as well as adjusting the FX send and return levels. Everything sounds great!