Can someone give me a hand with a DIY sub project?

Pick two of the above. Any two. You can't have all three.

What is your definition of small? Does that mean something like a 12" cube? Budget?

It's going to be real tough to get much from a 14" cube. You are pretty much limited to one 12" driver and a sealed setup.
You can only hope to get some monster driver, as in high excursion and throw a lot of power at it since you will need to EQ it down low.

You could try one of those new Dayton 12s and an affordable plate amp.

Since you have such a good budget I would encourage you to use a PR instead of a tiny sealed box. For example, this CSS APR 12, placed on any side of your 14" cube

http://www.creativesound.ca/details.php?model=APR12

The active driver could be down firing, and the amp placed on the back of the box. This will allow you to have a fairly low tuned sub that can handle a 500 watt plate amp

http://www.parts-express.com/pe/showdetl.cfm?partnumber=297-460

http://www.parts-express.com/pe/showdetl.cfm?partnumber=300-807

The best part about the APR is you can play with the tuning of the sub by adjusting the mass. The full 900g mass added will tune you to about 20hz at around -10 db. 630g mass around 25hz at -5 db. 450g mass around 30hz at -2 db. For output you're looking at about 110db from 40hz, down to whatever the tuning cutoff is.

agree. passive radiator is the way to go for a small enclosure, that will be loud, and tuned relatively low. the tradeoff is it costs a little more, but the op sounds like he has sufficient budget.

You can't cheat a few more inches to something like a 17" or 18" cube?

A 14" (external) cube built out of 18mm 13 ply Baltic Birch is going to have a usable air space of about 1ft^3 if you're really lucky (ie: skimp on the bracing some, have a driver with a small motor, and amp that doesn't have too much sticking in the box).
Edited by Stereodude - 1/13/13 at 9:01am

That amp will eat up a huge chunk of the interior volume in his 14" cube box. It wouldn't be my choice for a small box. The 500W Bash or Yung would be better in that regard.

Don't mount it in the cab, put it in its own box, placed as close as possible to the receiver. Run a long speaker cable rather than a long interconnect, greatly reducing the potential for ground loop noise. On-board amplification was foisted on us by the industry under the guise of convenience; we're DIY, we don't have to settle for what they give us.

Great idea except for this:

A plate amp in a box is far smaller than a rack mount amp, and can be stashed away behind other stuff. For that matter you don't even need to enclose many plate amps.

A Stereo Integrity 15" subwoofer will give you just about everything you want.

18.5 x 18.5 x 18.5" cube. Way under budget. $230 shipped.

Also, another one to consider would be the same size box with 2 TC Sounds Epic 12 subwoofers, dual opposed. $390 for both drivers. Would take a ton of power.

If you wanted something smaller than either of those, a single TC Sounds Epic 12 in a 15" cube would also work pretty well.

ETA: Could you make a sub to hide somewhere like behind the couch? If you made an odd shaped sub you could get a long port in there too maybe.
Edited by djkest - 1/13/13 at 1:15pm