John,
Consider Rythmik Audio's servo-controlled subwoofers for most accurate and lowest bass. Rythmik provides more test measurements and documents more design and engineering into their subwoofers than any I have seen.
Two and a half years ago for my home Hauptwerk system in a nearly 7000 cu. ft. Great room with cathedral ceiling, I choose the F12 (http://www.rythmikaudio.com/F12.html). It had the lowest frequency response without significant volume loss (http://www.rythmikaudio.com/F12_specs.html). Since an F12 is -3dB at 14 Hz, 32 foot ranks have consistent volume all of the way down to, and including low C. I have validated this on several 32 foot ranks. Since, I have had no issues and have enjoyed consistent performance. If I had it to do over again, I would still buy an F12.
The F12 still lists for just under $1000 USD, is sold directly online - all in less than an 18 inch cube.
Mark