Need help with driver selection

Sealed or resonant?

Sealed boxes can go all the way down to 1Hz, albeit very quietly.
Ported, passive radiator, TH, BLH, whatever, all unload quickly below their bottom tuning frequency, resulting in very little bass, but lots of distortion.

So, taking a 40Hz ported box and asking it to go to 20Hz isn't going to work. If you take a too-small box that's tuned low, things change. Lets say you've put a fairly big driver in there, and port tuning is 35Hz. Because the cabinet is a bit small for the driver, it starts rolling off at 50Hz. You can apply some boost to get flat to 35Hz, because the port and cabinet are still functioning in their intended passband - the driver won't be unloading, so you'll still get plenty of bass.

In short:
Boost below tuning is not good
Boost above tuning is fine, just be careful of playing sine waves at port tuning.

Thanks guys. The box is a curved side, very well braced 4.5 cu ft. It has a cutout for an 18 inch driver. I was trying to do this on the cheap but a friend gave me an iNuke 1000 with DSP. That coupled with the nice box deserves a good driver.

This Is for music only and I listen to organ music from time to time.

Sealed or vented?

If sealed, then get an 18" driver with the highest linear excursion that you can afford (I'd start say with this Dayton Audio RSS460HO-4 18" Reference HO Subwoofer 4 ohm or this Dayton Audio UM18-22 18" Ultimax DVC Subwoofer 2 ohms Per Coil) and use the DYNAMIC eq settings in the iNuke's DSP to start boosting where the subwoofer starts to roll off. Adjust the setting to ensure that no boost is being applied by the time the 0dB level is reached.

I'd go with the Ultimax in sealed and well stuffed.

In the EQ go for for a peaking 2nd order high pass, to
boost the LF and protect again rogue overexcursion.

You may find with room gain low bass EQ is simply not needed,
and the DSP is better used to control the higher room modes.

rgds, sreten.