I'm not really sure I see any reason to mod the s9. With apw5 and braiins os+ the s9 is way quieter than my r4 by running the stock fans on auto and the power >1000. I watch television in a room with 2 s9 at 700w and auto fans running >20%. The AFB1212SH fans actually run at a higher rpm and seem louder to my ears at the same settings as stock fans.
The afb1212sh fan pulls a lot less power IIRC