My expert hypothesis / gut instinct tells me there is no such thing as a “bad install” that would exhibit this type of behavior. Bad installs usually just don’t work at all. That leaves the largest variable being your setup…. so what is your exact setup? I’m not suggesting it’s to blame yet, we just can’t properly diagnose without this sort of information. Also, have you tried the simplest setup possible and/or different guitars / audio sources?
I play both guitar and bass through a large pedal board into their respective amps. I actually have a picture of the board at the link below. The full chain is described there but the salient point is that I am running several pedals before it and after it but there is a mix of true bypass and buffered pedals. Compression is before the H9s. There is a noise gate in the M5 after the H9s, but I’m experiencing this volume issue while playing.
I am playing an Ibanez RG Series guitar (passive, usually bridge humbucker) and a Music Man Stringray 5 bass (active, single humbucker). I notice the issue on both instruments, although it seemed like the issue might be slightly worse with the guitar, but it is present with both. I do have other instruments that I could try.
I have not tried guitar -> H9 only -> amp just yet. I will do that. However, considering this doesn’t happen with my other dirt pedals, my other pedals in general, or the other H9 algorithms, what about my setup could possibly be causing this behavior? I assume the mention of “different setups” is pointing to some input level/impedance mismatch issue? But I don’t seem to have that with anything but Crushstation.