I use an external master MIDI clock for everything (Lexicon MPX-1). In a practical sense, I need the extensive parameter control over the H9s, so they’re all slaves with OUTPUT=THRU. The XMT MIDI Clock seemed stable enough (in very limited testing). But there’s a lot going on inside an H9.
I try to keep the unneeded processing time to a minimum. I find the slaved MIDI Clock much improved with software updates, since my original PitchFactor purchase. Catches up to tempo changes nicely.
You’re mentioning USB, so I wonder if you’re using or slaving a computer clock. Surprisingly enough, that’s not the most stable route. Dedicated hardware performs much better for me; even an ancient drum machine (as a MIDI Clock source). There are standalone Clocks out there, but not necessarily a required purchase.
