I was running a 4 Tempest IB sub for the past 6 months, but it just didn't have the impact to match my room full of Legends. I did the IB in a line array, so I had a 22" x 75" hole in the wall covered by a frame and speaker cloth, so it was a stealth set up and nothing in the listening room. That part I liked, but I missed the ported sub. Then I realized what needed to be done.
I built 2 17^3 boxes tuned to 18Hz and stacked them in the spare room firing through the IB hole and hidden by the same grill. Only pushing 200watts to each sub. Fired it up tonight for the first time. Demoed a bunch of music and the sound is incredible; loud, detailed, punch you in the chest type bass. I do tend to listen at ridiculous volumes and this thing keeps up no problem. I would have to "guess" around 130dB @1meter as my old dual Tempest sub hit 127+ and this one is definitely louder. I'll have to break out the Rat Shack meter and see what it'll do.
I love my Legends with this sub. Insane volume and amazing sound. Can't wait to watch a few movies tomorrow....