[Josuah]

How much SPL are you lacking here? Or is it you want lower extension?

You can buy 24" tubes from your local concrete supplier. Look in the Yellow Pages. You can use multiple 4" ports, and software like WinISD will let you specify this, I think. Behringer amps are the cheapest for the watts, otherwise go higher to QSC or Crown or something. How many watts you need will depend on your design.

You probably don't need a high pass filter unless you intend to overdrive the sub. But if you do, I don't know any off-the-shelf gear that will let you set something below 20Hz. Phase can be taken care of either by a distance setting on your receiver, or an outboard processor; again something from Behringer would be cheapest. They also sell popular subwoofer parametric EQ units.

Sonotubes don't need any additional bracing other than the endcaps. You can do two layers of MDF for the endcaps if you want extra bracing.

Model the enclosure using WinISD or something similar. WinISD is free, some others are not.