I'd recently purchased a set of Sony APM X360 speakers for cheap at a thrift store; there are bottom-mounted downward-firing woofers, paper, with completely disintegrated foam surrounds.
For about the price of re-surrounding them, I found (via thefind.com and biznet.com) a pair of Pyramid car woofers of exactly the same frame diameter, ordered them up, installed them, and whoomp, now have fairly powerful deep bass. Yes, the port overloads and "speaks", and as with all such thick, hard cones, are more puffy-sounding than rock-solid. They aren't really expensive so they will blow if pushed too hard, but what I'm getting out of a pair of 8" woofers (delivered for around $35.00) is perfectly acceptable.
I've also found next to nothing on the net about these Sonys. They have the square, honeycombed midrange (but obviously not the square woofer of more expensive models). There is only one very small sheet of damping material inside, and the cabinets are not very well reinforced (tap the area between the mid and tweeter with the tip of a screwdriver and you'll hear resonances). Worst, they have cone tweeters...
