Sounds like a phase issue to me. Parallel loops run both signals (dry and wet) simultaneously, at the mix ratio you set. The signals are supposed to remain in-phase with eachother, but if for some reason the phase of the wet mix is say flipped 180 degrees, phase cancellation will occur and result in a scooped tone. Not familiar with the loop on your amp, but I've had the issue in the past with early JCM2000's.
In all likelyhood there is nothing "wrong", effects in parallel loops are just meant to be run in kill-dry or 100% wet (on the pedal, not the amp). This way the mix on your amp only mixes in the affected signal, otherwise you are also mixing in an additional version of your original dry signal, causing the phase cancellation. I've included the killdry instructions from the manual below, hope they help!
Killdry – Send/Return Loop
If you’ve connected the H9 using an effect send/return loop (either your amp’s or a mixer’s), the H9’s Killdry feature can be enabled.
When Killdry is enabled, the H9 does not send any dry signal to the outputs; only the Effect. Note that when Killdry is enabled, Bypassing will mute the Effect rather than Bypass it when DSP Bypass is selected.
Killdry’s factory default is disabled. It can be enabled in the [BYPASS] menu of the System Mode. Here’s how:
- Press and hold the HOTKNOB button and Right Footswitch simultaneously for a few seconds to enter System Mode.
- Turn the Knob until [BYPASS] is displayed.
- Momentarily press the HOTKNOB button.
- Turn the Knob until [KILDRY] is displayed.
- Momentarily press the HOTKNOB button.
- Turn the Knob to select either [YES] or [NO].
- Press and hold the HOTKNOB button and Right Footswitch simultaneously for a few seconds to exit System Mode.
