Two Crappy 10's or one good one?

In the past, I had two Sony 10" subs rated for 600W max each. I powered them with an old Rockford Punch 60ix dual channel amp. In my new(er) car, I can't put a box in the trunk, because it's too well insulated from the inside of the car. I've designed a box for one 10" sub and I've layed it all out to build from fiberglass. I currently only have my crappy 10" Sony Xplods. In the past, they didn't sound terrible, but they were in a bandpass box, which they weren't made for. They seemed loud enough for the most part. I can only fit one 10" sub if I build this box, it will be about .75 cu ft. I'm wondering if later on down the road (when I have the money), I decide to replace the 10" Sony sub with a good 10" sub, will I be able to have get as much volume out of a good 10" sub as my original setup with two Sony 10's?

Before you go buying just any 10" sub, actually learn about which subwoofers work well in that sized box. http://www.diysubwoofers.org/ can get you started. I would buy a single 10" if you want sound quality. If all you want is bang, go with two. (in general)New Project: 2003 Pathfinder