I would stay away from passive radiators. My understanding is that they do essentially the same thing as the port on a reflex cab. They are not necessarily any louder or better. I agree with Jim that there is a fair amount of science that goes into designing a good bass cab--which is not to say that every cab out there is a good one....
If you are building it's a good idea to "tune" your cab to the speaker you will be putting into it. There are a number of calculators for this on the internet, such as the one below.
I haven't done it for a while, but there is also some free software you to help you design dimensions. I can look for it if you are interested.
I think Simon is right that there there isn't necessarily much difference between the woofer in a PA and a woofer in a bass cab. Lots of Eminence speakers are used in both. That said, if you are bulding a cab you want to get one with published specs so you can tune your cab to the speaker you want to use.
Another thing to watch out for is that small and cheap speakers may have (a) a low power rating, and (b) too short "excursion" (a.k.a xmax--the distance the voice coil can travel to move the cone) for bass.
Think of it this way. Bass sound waves are much bigger than guitar sound waves. To produce the low E on a bass you have to move a lot more air than you do for a low E on a guitar. All things being equal a small cone will move less air than a big one. Higher Xmax will help a bit since it will allow the cone to move farther. Exceed the Xmax by turning up the volume too high, and the voice coil can jump out of it's spider or housing. That's never happened too me, but what has is a melted down voice coil. This happens because speakers have a maximum volume that they can produce. However when the speaker reaches that point, I might still want more volume--more power to a speaker that has reached it's maximum volume results in heat rather than sound...and, in my experience, also a bad smell then silence.
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