linkwitz transform circuit

You can boost below the driver cutoff--that's kinda the whole point. I used to run subs about an octave and a half below their nominal 3dB point (from around 45Hz to about 16Hz). The trick is to precisely balance the circuit against the driver for flat response. Just keep an eye on Xmax and available amplifier power and you'll be fine.

Grey
 
Well you can go below the Fs but you may then need a notch filter... also Xmax gets crazy ... you could go down to 10Hz if you want but SPL will suffer alot in order to stay inside Xmax and Pd.

If you post the t/s parameters of the driver you will be using and the rating of the amp we will be able to give you more information.

While your at it you'll probably also want to compensate for baffle diffraction loss.