DIY sub under $250 possible?

Sure, you could get an Infinity Reference 1060w for $55 off Amazon. The price went up recently on the Infinity drivers. I have seen them for even less. Then get one of the 250-300w plate amps from Parts Express for around $130. Build a ported box with the dimensions you listed above. You will need to find a plate amp with a highpass filter to prevent over-excursion of the driver. You should get decent output into the mid 20's. And you come in under $200 for parts which gives you another $50 to spend on wood, glue, screws, etc.

I agree with the Infinity rec

http://www.amazon.com/Infinity-Reference-1060w-1100-watt-High-Performance/dp/B0028APNZ2/ref=sr_1_1?s=electronics&ie=UTF8&qid=1361805307&sr=1-1&keywords=1060w

Build the box to your dimensions (20x20x15), stick this port in it

http://www.parts-express.com/pe/showdetl.cfm?partnumber=269-165

And use this amp

http://www.parts-express.com/pe/showdetl.cfm?partnumber=301-504

1060w? why not a 1260w for couple bucks more
http://www.sonicelectronix.com/item_18916_Infinity-Reference-1260W.html?SearchClickout%5Bquery_id%5D=16919203&SearchClickout%5BIgnore%5D=1

Can save on the yung brand plate amps if read this thread. Know how to defeat the boost could save by buying a boost model and defeating it.
http://www.avsforum.com/t/1410846/yung-plate-amp-initial-impressions/90

Amps from PE
http://www.parts-express.com/term/yung?srch=yung

search for dual opposed infinity 1260 / 1262 threads in the sub forum

enjoy!

He would need a bigger box to fit a bigger port to avoid chuffing with the 12. The box I spec'd accounts for the yung 30hz filter to avoid chuffing while giving solid 25hz output.

Now if he wants to go DO sealed thats another option.

The problem with DO is that it adds $55 to the total cost of his already tight budget. I think he gets more bang for the buck by going vented. A dual opposed configuration does offer some advantages though. You will get lower cabinet vibrations, and lower group delay which in theory should make the bass sound tighter. It comes at the expense of needing more power to achieve maximum output, more power costs more money, and you won't get as much output in the lower frequencies as you do with the vented design.

If you do go DO, then get the 1062w model instead of the 1060w. That way you can wire those to a 4ohm load. With the 1060w's, you would be at 2ohm or 8ohm. 2 ohm will be too low for a plate amp, and 8ohm provides less power.

I attached my winisd files for you to use. Also make sure you are using WinISD Pro ver 0.7. That is the latest version and will work best with Windows 7 or later.

Infinity drivers.zip 2k .zip file

WOW out of stock on the 1260w i knew i should have got 4 instead of 2. I hope they get them back in stock. also the price is higher now?

The status said "more on the way" when they were down to 1 left in stock, so I assume they will be getting more. I also suspect the price will come down after they get more stock. Maybe not right away, but Amazon's pricing is usually pretty dynamic as competitors increase and decrease their prices. Even if it doesn't, their listed price of $59.99 still makes it one of the best bargain drivers available.

You would have to change your box dimensions and use this port tube http://www.parts-express.com/pe/showdetl.cfm?partnumber=268-350, or you can build the port with an elbow in it and use the following pieces:

2 x http://www.parts-express.com/pe/showdetl.cfm?partnumber=268-372
1 x http://www.parts-express.com/pe/showdetl.cfm?partnumber=268-369
1 x http://www.parts-express.com/pe/showdetl.cfm?partnumber=268-368
1 x http://www.parts-express.com/pe/showdetl.cfm?partnumber=268-328


You can also save some money and buy PVC pipe. Some people will use a router to round over the hole on the outside of the box to create a flare, and glue PVC pipe to it. I have also seen people take plastic tube, heat it up, and press it over a hard sphere such as a metal bowl to force the plastic to bend into a more rounded shaped end. If you don't want to do either of those, I am pretty sure you can buy just the flares and attach PVC to it. I am not 100% sure that the ID of the PVC will match up perfectly with the flares. Maybe someone else can chime in on that. I know you are on a tight budget, and all those port pieces add up so I thought I would mention it. Another option would be to use a slot port instead of a round port. A slot port would probably cost you nothing, since you would likely have enough material left over from a sheet of MDF to build the slot port. If you want to compare the two in winisd, just click the circle in the vent section of your design and it will change to a square. Enter the internal dimensions of your slot.

Also make sure you are factoring in your box volume correctly. You can use this site http://www.hometheatershack.com/forums/enclosure-volume-calculator/#axzz2Kd0ZRoOV to calculate your box volume. You can probably subtract 0.25 cubic feet for the driver and amp. The site has calculators for determining bracing volume and port volume.