Correct me if I'm wrong, but it looks to me that you're looking for a cheaper 15" unit to work in reasonably compact boxes? Compact for a 15", that is. Eminence have some that don't look to expensive - all from the 'American Originals' series (pressed steel chassis). The Gamma 15 will work in a box of 80 liters (~2.86 cu.ft.), tuning 42Hz, -3dB at 60Hz. In a bigger box of 118 liters (~4.2 cu.ft.) it makes 50Hz with tuning at 44Hz.The Kappa 15 LF from the same series offers 42Hz in a box of 130 liters (~4.64 cu.ft.), tuning also 42Hz. In a smaller box of 100 liters (~3.57 cu.ft.), tuning 40Hz, it reaches ~47Hz.
The Kappa 15 has a 3" long throw VC and can take a lot of power, too. So I bet that one can play more than loud enough without distortion.
Just some suggestions - maybe the above units could be made to work well with some already existing Pi X-overs, perhaps with some small modifications? But Wayne knows about that :-) Good luck with your project.