Personally i never seen the Slipstream (now Glidestream) as a good CPU heatsink fan, some years ago, the Slipstreams were more marketed into case fans, and the Kama Flex were like the high quality fans and worked well on heatsinks, but as soon as the Scythe Tower coolers came out, they used Slipstreams..... and now glidestreams.
To me one of the best heatsink fans are Scythe Gentle Typhoons, reviewed by SPCR on Fan Roundup #6: Scythe, Noiseblocker, Antec, Nexus, Thermalright
, but there are no PWM versions, but Asus motherboards are very flexible, specially on the CPU_FAN header, if you enter the bios and change the Q_FAN from AUTO to ADVANCE, it will change from PWM to Voltage control, this will allow you to use a Scythe GT.
There are different models on scythe GT, mostly as the slipstreams, its their top rpms, SPCR tested the 1450 and 800rpms, both performed well for its brackets, i own 1850/1150/800/500 rpms versions, and to me, the Scythe Gentle Typhoon 120mm Case Fan- 1150RPM
is the best for a CPU coolers, as it still dips down to 350rpms under FanXpert2, and you can ramp it to around 1200rpms depending on your needs. I leave you my FanXpert2 test on it,
Now on PWM, SPCR didnt like much the Noctua NF-F12 tonally, and the NF-S12 is more a case fan. Noiseblocker has a pretty good tone fan on the M12-S1/2 but their pwm design is a little complex, specially because the lowest you are able to drop the PLPS is around 800rpms, weather this is audible to you.... only you can say, personally it was a little to high for me for the lowest setting, but it all depends on your conditions and what you want to achieve.
Good luck with your choice,