Noctua's low noise adapters are just inline resistors which reduce the voltage available to the fan so it spins at lower rpms. They're meant to provide a lower-rpm (and lower noise) option only for motherboards which do not support 4-pin PWM fans. If the mobo has PWM fan control (as every current Asus mobo does) then it can already fully control the rpms without these adapters and lower-rpms (lower noise) can be configured through firmware/software settings.

They're basically useless for you, aside from keeping around to use on other systems or as spare parts.

I have a Noctua NF-A14 for my rear exhaust and two Noctua NF-F12 on my Corsair H105 liquid cooler. The main reason I buy Noctua or Cougar is to run them at max speed. Anything over 26db annoys me, I can hear it and most other fans are in the 30's and in that case I can see using the attenuator or reducing the speed via the fan controller but I don't hear these guys at all. They have such good airflow and very low noise. No reason to reduce the speed. That's why you pay the extra money for them. I have the fan controller section that controls them off, and just set em at full speed.

Only advice I can offer concerning Noctura is to make sure it's the model you want. Bought a couple of NF-S12A, 107 cfm, 17db. I was happy until I realized I need a high static pressure to push air through the radiator. The NF-S12A have only 1.1 mm H2O (static pressure). Had to buy another set, the NF-F12. Lower airflow, 93 cfm, higher noise, 22db but 2.8 mm H2O. much better for a radiator. NF-S12A make a good case fan but not for cpu or radiator.