Originally Posted by capecodorthopod
I find below 20 hz useful, it just costs much more $$ to do it with sufficient output at relatively low THD.
I also disagree with nube's assessment of the Empire's extension. In my 3100 cu ft sealed bedroom a single Empire is quite strong (110 dB) to 16 hz which is in line with it's CEA #'s (98 dB @ 2m half space.)
It's my analysis of the results of Ricci's testing. Check out the Empire's measurements here:http://www.data-bass.com/data?page=system&id=50&mset=48
Pay particular attention to the basic response, where it drops like a cliff after 20hz (as it's designed to), and spectrograph (see how high the Empire's output is centered?).
Compare those graphs side by side in multiple browser windows with, say, the Rythmik FV15HP (A ported, single 15" sub) in 1-port mode, measured with the same criteria:http://www.data-bass.com/data?page=system&id=51&mset=49
It just slays the Empire down low (which you wouldn't think it would, being a ported sub vs. the Empire's dual opposed sealed design), and is much more consistent across the bandwidth. Granted, the FV15HP is $300 more, but I think you get about double the sub where it counts, from 12-40hz. And, it's not so peaky in its response, so chances are it's going to integrate with your room and system better.
At the end of the day, to the OP I still say DIY, especially since you have some experience building boxes. Sealed boxes are ridiculously easy to build. Get a friend to help, and take your time. You'll get worlds better results for your money.