I second Danny's suggestion of subwoofers from Rythmik Audio. I have owned and used a Rythmik F12 for 4 years:
http://www.rythmikaudio.com/F12.htmlF12 is a 12' sealed direct servo active subwoofer with a 370 watt RMS internal amplifier. I chose it primarily for the advertised extended low frequency response: 3 db down at 14 Hz:
http://www.rythmikaudio.com/F12_specs.htmland for the Direct Servo system because it accurately reproduce's each sample's sound.
Walking down the bottom octave of 16 foot ranks (for example at A4 = 440 Hz - 65.4 Hz down to 32.7 Hz), and 32 foot ranks (32.7 Hz to 16.4 Hz), I have confirmed that the F12 evenly and solidly reproduces every pipe. For new listeners and players of my Hauptwerk system, I have done this to demonstrate that every low end sample is being accurately reproduced.
Given my years of use, if I had the chance to choose again, I would choose a Rythmik subwoofer again.
Mark