Subwoofer questions

As far as not wanting to do DIY above $300, the performance per dollar of DIY gets exponentially larger as compared to commercial, so rather than fearing you wouldn't match the performance of a more expensive commercial sub, you'll actually be far exceeding it.

But to answer your question, you can go with a Dayton Quatro 15" and the Dayton 240 watt plate amp with built in 20hz highpass from Parts Express. Aim for 250 effective liters with a 6" diameter port that is 15" long, and you'll be pretty good above 20hz. Quatro isn't suitable for downfiring, so you'd want to build a box as opposed to a sonosub.

Quatro 15
240 watt plate amp

The box can be any shape that is convenient for you to do. The volume is important, not the shape. A diy sub will far exceed what you're going to get from a commercial sub. One reason is because you do want bracing, the more the better. Most people do the front baffle at least 2 layers thick of 3/4" mdf as well as the rest of the box.

You can also use Baltic Birch plywood and get away with one thickness. This stuff is lighter and you can just finish it with stain and varnish when you're done if you like the look of birch. The birch ply is a nice smooth sheet with an attractive grain. In the long run it may save you money over using mdf and veneer.

If you can afford to spend more money it would behoove you to do so. In the long run you'll more than likely end up saving money becauase you won't be craving more. Once you hear what a real sub will do for your home theater system you could end up wishing you went ahead and spent a bit more money. Even with the little sub Steve suggested you'll be amazed how much better it is compared to your current woofer.

There's a thread here called DIY gallery as well as a thread called Mission Accomplished over at HT guide. Go down to DIY subwoofer showcase there and there will be another thread with alot of pictures and descriptions like the one here at AVS. If you need any help on port size and box dimensions just ask. If you want to come close but slightly different to a sub you find in one of the threads you may need to change the port length etc.

I am in the same boat as you as far as wanting to stay below 300 bucks. I am dumping my h100. What I am considering is hooking up a bash 500 amp to either the
12 inch TC-1000 when it goes on sale for ~159 or the
15 inch Dayton RSS390HF or RSS390HO when they go on sale for 139+f/s
That should be just slightly over your price range and from what i have heard a monster of a sub for the price.

I dont know if you know much about the enclosure design but the program winisd pro alpha can be very helpful and is free.