Thank you all for the replies. I now have several more potential solutions to try.
If you connect it up as a DC fan instead of PWM (ie. only 3 wires) and use a very low voltage then what is the minimum speed you can get?
I plan to try it and report back.
i vaguely remember MSI having some minimum duty cycle implemented in their BIOS. So, no matter what, some minimum fan speed determined by that threshold will not be undercut, regardless of BIOS settings.
So you might end up hooking the fans up to 7v or 9v + PWM.
My data indicates that my MSI BIOS might have a minimum PWM speed of ~800 RPM on the CPU fan header. I wonder if the PWM system fan header has that limitation. I plan to try connecting my next Nexus PWM to the PWM system fan header as an experiment.
The second part makes sense, but I don't know whether to raise the voltage of the 12v negative lead or to lower the voltage of the 12v positive lead. The electrical engineer in me thinks that only one of these two options will result in a working PWM contol system.
I plan to try PWM fans in the following configurations: 5v 7v and two 6v. I may try 9v too if those are too slow; I have the Noctua heat sink's stock resistors handy (but I don't know where my Zener diodes are). If the MSI BIOS minimum speed is indeed 800 RPM then I expect to see the same RPM at many different voltages.
If all else fails I will look into BIOS mods... in 2 years when my warranty expires;-)
500-2000 RPM (+/- 10%)
so technically it might only get down to 700 rpm even with lowered DC voltage, or proper pwm.
I can't justify buying a proper PWM controller at this time; the money could be better spent on other things.
I will, however, try a lower voltage as I mentioned earlier in this post. I'd gladly try almost anything. I'd be 14% happier with 700 RPM than I am now, with 800 RPM.