mH coils are too unwieldly to wind as air core coils. Look at DigiKey and see what you can buy as an ready made "inductor". Frankly, I think you are on the wrong track. Filters like this are usually inserted ahead of the power amplifier which drives the speaker. That way, you can use "active filters" like in the link I gave you.
You need to use a core. That site's calculator fits for air-core inductors. And do not use a 30 AWG wire for a subwoofer, the inductor will have too much parasitic resistance.
A subwoofer lowpass filter is usually active and feeds the sub-woofer's amplifier. Inductors are not used.
Usually a second order or third order filter is used, not your gradual and droopy first order filter.