inserting units inside the duct may reduce the acoustic side, however, probably increase the dead unit risk as S9 is just like any other electronics. You have to consider ESD effect and little jolts from dry air season. DUCT does reduce noise that is for sure tho but please prepare for some hash PCB loss or control module loss in a longer term.
S9 fan speed is manually changeable, however, if you do not have heavily air conditioned room, please be very careful with manually adjusting the fan speed. Fan speed adjustment can bring you 2 possible benefits that is Noise and Electricity Consumption. There is a con to that of possibly damaging the S9 if something goes wrong. If you are in USA/Russia/Ukraine, repair is locally available to you, however, its not free.
If the noise is the problem R4 is the way to go
good points the s7 was able to do this, but due to the smaller chip size the s9 is a bit more delicate then the s7.
If you go the route of ducting an s9 it could damage due to static charge.
the line of the duct is static resistant temp plastic the center is fiberglass also static charge resistant,
but the outside of the duct is aluminum so if you ground the outside it would be less likely to spark static electrical discharges.
I have used ducting with s7's pretty much perfectly for all of last winter. Yet to do it with an s9
