If you like diy, then maybe give a horn loaded/transmission line sub a go? Cheap enough that it won't break the bank if it doesn't work out. I was headed in this direction, but the shipping costs of the components killed the project.

http://www.billfitzmaurice.com/

I was going the Table Tuba route, but if you have room for 2 800's, I'm sure you could fit a Tuba HT or maybe 2 HT's stacked.

http://www.billfitzmaurice.com/THT.html