Someone's probably going to tell me that this is a bad idea, but...

[Mr Peabody]

You will need a super high efficient woofer or find space for an outboard amp. Although the channels may bridge, a head unit just doesn't have enough room for the size power supply to generate ample power no matter what is stated on paper.

I would also suggest maybe a single large driver if you can make the space work, like a 10". A 6" just doesn't ring "sub" to me. Could provide some fun midbass punch though.

Does the head unit have a built in crossover? If not, there's another thing to consider.

A sealed box would work best in small spaces but sealed boxes require more power. Also, less math to do with a sealed box, just find a woofer designed for sealed box and match the minimum cubic feet. With a ported box you'd have to figure your cubic feet, pick a woofer and find a program or formula to tell you what size and length of port you would need.

It's been a long time since I've done any car audio. If Infinity still makes the Perfect sub series they were very good in sealed enclosures. One of those with a small Rockford bridged and you'd be slamming. If budget is there I actually like Precision Power amps better. Then again, due to space and heat, for your project if considering an outboard you should really use a digital amp.

Why not put a box in the bed with four 15's and run the port into the cab

[Ged]

Believe me, I know that what I've posted above is an answer to a question that almost nobody is asking. It's been bounced around over on one of the Crutchfield forums a few times over the last several years by people (that probably aren't audio DIY'ers) asking about what most people think of as a traditional sub (10"/12"). I'm thinking of something that's more in line with what the head unit is actually capable of driving.

Based on my reading, I think the "sub output" is really somewhere between 30w and 40w rms. I'm going to assume that it's on the low side of those figures. The drivers that I'm looking at are all rated from 30w to 60w. As for efficiency, could the arrangement that i've described be considered a line array?

The head unit does have a HPF/LPF, both with settings at 50hz, 63hz, 80hz, 100hz, and 125hz. It also has gain settings and bass boost.

I'm not really expecting a lot of "thump" out of this kind of thing, but it would be nice if it was clean and smooth at decent volume, while taking some of the low frequency load off the door speakers. If this were fun and maybe even economical, that would just be a bonus. It would certainly apeal to my "road less travelled" nature!