Don't remember the exact specs OOPS, but its running on a 260 liters BIG sonotube tuned to 21Hz. Thanks to my wife for letting me do whatever I want in my listening room!
Oh, regarding the "why", because I wanted the most BOOM I could get (total SPL this is, not 60Hz punch) for my money.
I can get around 110dB at 18Hz in my small room, not that I run it at that level to hear music! ;)
Sony CDP C315 (1 bit Pulse DAC), Margules ACRH-1 Hybrid Integrated Amplifier, Margules ADE-24 (Black Box), Margules QR1 (Line Conditioner), Klipsch Heresy Speakers (the closest thing to a "horn bookshelf" you can find), Tempest DIY Subwoofer