Currently running 2 HDS 215s (600rms, 15 inch) subwoofers in the back seat of a mustang convertible. Not much of an improvement over my previous Sundown E-12s. The problem I believe is the box. being obviously small to fit back there. Around 2 cubes per sub. 

Would I be better off getting a new box to put back there, say the QBOMB15VL where there is 3.85 cubes (4.2 total) for A SINGLE 15 rather than my 2 15s now.
Also found on ebay a 4.0 box (5.33 total) tuned at 32hz.

Also I could upgrade the subwoofer to something that can take my 1700rms amp better. Any help appreciated. 

Edited May 7, 201510 yr by Dangercdv

Space first...

Max width is about 38 inches. Depth and width about 20 max. I think it would sound a lot better with a single 15. Would it be as loud?

Max width is about 38 inches. Depth and width about 20 max. I think it would sound a lot better with a single 15. Would it be as loud?

One woofer in a proper enclosure can sound much better than two shoehorned into one that's too small.

Well no way you can fit a pair of 15's in any logical manner so it won't be hard to improve.

ok, so I can get a 4.0 cubic box at 32hz, might upgrade the subwoofer too. I don't care for it to be perfect, just want some SQ improvement and to hit the lows better.. 

Then start modeling.

Well, I found a good one on ebay. its $150, and im in military college right now. Very little free time, no tools and no place to store anything. And being far from home im very limited with alot of things but shipping something here is about all I can do. I want to sell my box and maybe both 15s with it and buy a new subwoofer. Maybe make $200 for my current subs and box

I would save up more and buy a custom enclosure rather than prefab, it will be worth it in the long run. Try Thumperr on here:

http://www.soundsolutionsaudio.com/forum/topic/63299-enclosure-pricing-for-2015/

I would keep one sub and get a serious box for it. You would have one sub to make some money for the box or for a better sub later.