I imagine that the amount of heat generated (and therefore cooling required) will also depend on how many expansion cards are installed in the H9000.  For those of us with stock 9000’s and no expansion cards, a lower fan speed may be fine.  People with fully loaded 9000’s may need the maximum amount of cooling possible.  That’s why I agree with drxcm and hope Eventide can provide fan speed management via the OS.

(FWIW, I have installed quieter Noctua and similar fans in various older effects in the past and it’s been fine, but my use of the gear and how I have it racked may not be the same as you).