you should be able to take the fans down to 200rpms or ramp them to 800rpms.
Looks like there is no PWM in Asus Sabertooth Z87 CHA_FAN (manual http://www.asus.com/Motherboards/SABERT ... sk_Manual/
page 1-30), so I doubt it will be able to take it down to 200.
I think you are confusing PWM and controlling fans, specially how Asus motherboards work. FanXpert2 can control PWM fans and 3pin fans, you just gotta understand how it works.
CPU_FAN is a true PWM fan header that can control 4pin pwm fans, but it can also control 3pin fan via voltage control if you enter the bios and change the Q-FAn from auto to advanced.
CHA_FAN1, 2, 3 are fake 4pin headers, in other words they are voltage control headers, meaning you can control standard 3pin fans, fanXpert2 just undervolts the fans according to what he tests.
In the case of Antec True Quiet 140, its 3pin fan not PWM, the graph that i placed was a Antec True Quiet 140 ran on a Asus Maximus VI Gene on CHA_FAN2 (renamed as TEST FAN) under FanXpert2, in the table you can see the fan can be dropped close to 200rpms, if you see the little square on the bottom right you will see the TEST FAN is running at 200rpms. You can also check SPCR Second 140 mm Fan Roundup: Antec, bequiet!, Corsair, Scythe
, where you can see their table they also are able to idle it that low.
To get them that low you just need to install the Antec True Quiets to CHA_FAN 1,2,3, install AI SUITE III, enter FanXpert2 option (on the top), and in the middle left part you will see the test button, run it and it should test the fans and drop them to 200rpms.