The best 'available' drivers available in that price range are the Dayton RS 10" and 12" subwoofers. These have extremely linear motor designs comparable to the late Infinity Kappa Perfects. Note: it is not easy to get a ported design in a very small cabinet, as a proper port with low compression takes up a substantial volume by itself. But, you can use a trick I call a 'helper' port. You need a high pass subsonic filter like the O Audio 500 amp has, however. to use this trick. What you do is use smaller than ideal ports (standard round ports of course) to save internal volume, and assume your true extension is about 6-7Hz higher than the actual port tuning frequency. So, if your port is tuned to 25Hz, assume compression free linear outpout extension is really at about 32 Hz. This is because this far past the center tuning, the port velocity will be low enough to not be a factor. However, you need to engage a subsonic filter to start at about where the true useful extension begins(at 30-32 hz in this example) to prevent substantial excitation at the actual port tuning frequency(thus avoiding port noise, etc.). I call this a 'helper port'.
I used this technique on my computer stereo subs to keep the volume from becoming too large. Tuning is at about 25Hz, but true compression free output extends to about 30Hz(anechoic), and I have a subsonic filter engaged under 30Hz.
-Chris
