I changed them out for NF-A15 HS-PWM chromax.Black.swap to eliminate the brown fans.
I already have the two fans, but as mentioned cannot use the rear fan (which is actually the one closest to the front of the case) in push because of my RAM height. Consequently that fan is currently in at the front of the D15 in pull.
My thoughts were that I could replace the NF-A15 with a NF-F12 PWM chromax.black.swap which has the same RPM characteristics as the NF-A15, but would fit above the RAM in a push configuration.
As already said, it's not worth the effort. If you have 171mm CPU clearance and 52mm center CPU to RAM you could use a Thermalright TRUE Spirit 140 Power instead of D15 and have as good or better cooling with slightly less noise. Another option is Le Grand Macho which only needs 48mm center CPU towards RAM and cools as good or better than D15 with a little less noise. TRUE Spirit 140 Power is about $50 and LGM RT is about $80.
Phanteks Owners ClubWays to Better Cooling
i7 980 @ 3.55GHz =PH-TC14PE w/2x TY-143 fans =Crucial Ballistix 3x4GB =GA-X58A-UD5 =ASUS GTX580 DirectCU II =Enermax Modu84+ =Define R2 w/3x TY-140 case intake fans; all PWM controlled by CPU fan socketPhanteks Enthoo Primo MoBoRampage III ExtremeCPUi7 980X @ 4.0GHz =R1 Ultimate w/2x TY-143 fans =Dominator GT 6x2GB =Rampage III Extreme =ASUS GTX580 DirectCU II =TX850 =Enthoo Primo w/ custom castor base