@NickM: Thanks! Yeah, already found the lm-sensors package. That's what I thought you must have been using (needing an operating system and scripts setup). FYI, I believe the values is 0-255 for PWM. With 255 = 100% PWM duty cycle' and, 0 = 0% PWM, which means the fans will as low as they are programmed to operate at, but not "off". A "PWM fan" has an onboard circuit that monitors the PWM duty cycle and is pre-programmed to spin at X RPMs with 0 PWM being sent.
With that said, I experimented and found the 50% PWM duty cycle that the Optimum setting of the BMC on the X10SRA-F to be sufficient for noise. Still very audible; but, it is going in the basement. I like the BMC controlling the fans in the vent that the operating system freezes or something. I have two fan headers connected to the 3 PWM fans in the mid-plane, and the rear 2 connected to what is considered to be "CPU Fan2", which duplicates CPU Fan1 header. This allows the system to spin up the rear fans in the need for more CPU cooling, which works perfectly with my heatsink setup using the stock fan shroud (i'll post pics later).
Regarding the replacement of the ATX...
Yes, that's what I am saying. I have a few "SQ" models of the stock PSUs and while they are quiet, yes, they still have a low-groan to them - they aren't 100% quiet/silent. I have a few 740W Platinums and only 1 is "quiet", the other 2 have a low-groan to them as well - a tad higher than the SQs. But i do have 1x 740W Platinum which IS quiet - as long as there isn't any real load on the CPU. Running mprime on the CPU eventually makes the 740 Platinum audible a little, but the CPU fans are already kicked in and pretty loud at that point.
Alternatively, just about any decent standard Gold or Platitum ATX PSU is silent - or at least completely inaudible.
As others said though, once you replace the PSUs with SQs, or even the Platinum versions, the chassis' stock PWM fans are the next thing people find annoying and replace all of them with quieter fans, or even PWM fans.
So, mixing in silent PWM fans with a standard ATX PSU gives you a silent 24-bay chassis you can stick under your desk. My problem is if you had 24 HDDs spinning, that would need a LOT of cooling which I'd be concerned with by replacing the PWM fans.
Me, personally, I like the umph (static pressure) the stock PWMs have and will continue to use them in my setup. I'm using StableBit's DrivePool. ZFS and Freenas and alike are nice and have very high bandwidth; but, they spin up all HDDs. What I like about DrivePool is I am able to spin down all HDDs. Each 4 and 6 TB drive pulls about 8W from the wall in my setup, so that's 14x 8W = 112W. When I command all HDDs to spin down, my system goes from 185W down to 74W idle at the wall. So one could replace all PWM fans since only 2 or 3 HDDs would be spun up at any given time. It's when all HDDs spin up is when I'd really want all fans.