Thank you! I'll be changing my motherboard - mostly for other reasons, but it looks like the one I want to change it to has 4X4-pin PWM fan headers - one for the CPU fan and 3 for case fans.
Just be careful into what motherboard you chose, most motherboards only have one true 4pin PWM fan header, most of the time is the CPU header, the rest even if they 4pin phisically, they are 3pin in disguise. There are some that do come with 2, like MSI (CPU_FAN1 and CPU_FAN2) and AsRock (CPU_FAN and SYS_FAN1) in some of their line of motherboards. Some say that intel mobos have all true PWM, i cant say this for sure, as in my mini itx build i only used 2, and both were PWM fan headers, but never really tried an ATX mobo with all the fan headers. This is why would be best for you to chose the motherboard depending on the features you want and the price you willing to pay, and just use those PWM fan splitters if you want to go full PWM fans.
Which brings up another question about 3-pin DC fans in 4-pin PWM headers, but that's a question for another subforum.
On Asus motherboards, they only have one true PWM fan header, CPU_FAN (CPU_OPT is just a mirror), but on asus motherboards you can enter the bios, go to fan control and change the Q-FAN from AUTO to ADVANCE, and it will make the PWM fan header into a voltage controlled fan header, like this you can use any 3pin fan on it and control it via FanXpert2. Other motherboards i havent tested if there is a feature like that, but in most cases there is only 1 true pwm header and the rest are 4pin fakes, but its not a big deal either, just gotta understand how is your motherboard design and buy the correct fans for each header.