BIOS control correct.
These Arctic Cooling PWM fans are supposed to have a range of 600 min to 2000 max RPM , but on Shuttle XPC Motherboards I suspect the logic programmed into the I/C's that control the fans are specifically for the stock one supplied and installing another with different spec's often doesn't allow the minimum RPM's to be reached.
You are often able to get lower RPM's if using Speedfan or even Shuttle's xpc tools with replacement fans rather than the BIOS control settings, though I prefer to let the BIOS control them rather than another layer of software/complexity/problems, etc.
It's good to read that you are able to use these two off one header and which both have a 0.13A current draw. Was the original 92mm a Shuttle badged AD0912UX-A7BGL?