Well, I am almost convinced that you have selected the best subs for your current box, output requirements, and amp. It seems the only thing that's missing is those low frequencies in the area of 40Hz, which are lacking due to the small box size.
I've thought of another option which will allow you to keep your current subs, box, and amp:
Audiocontrol Epicenter With the Epicenter and its frequency and sweep adjustable bass boost, you should be able to dial the extreme low bass frequencies back into your system. It'll probably take a while and pleanty of experimentation to get it dialed in just right, but I think it might be the ticket.
I haven't used one myself, but every report/review I've heard on the Epicenter has been very good.
BTW, I'm also assuming you've got the boxes heavily stuffed with polyfil, acoustastuff, or equivalent. It'll make the subs think they're in a bigger box and help to improve the low bass frequencies.
