You can get I would say as much as 7-8 dB more at 20 Hz. This is just going off memory, but I have modeled the Tempest more times than I can count.
As to what frequency you should tune the box, that's completely your choice. I don't have a Tempest, but a Dayton Quatro 15", which models within a gnat's eyelash of the Tempest, just has less Xmax.
Anyhow, I have mine in about a 270l enclosure tuned to 11 Hz. Why? I felt the slow rolloff would be complemented by room gain and produce a flatter response than if I designed the box to be flat to 20Hz without room gain. It also pushes the peak port air speed frequency and group delay peak below the audible range, so in terms of sound quality, my box sounds extremely close a sealed version. If you want the most output, you typically tune to the Fs, which I think in this case is 19 or 20Hz, right? If memory serves, 230l is a bit larger than optimum for that woofer, so you may want to tune slightly lower, say 18Hz, or you could end up with a hump in the response around the tuning frequency.
Aaron Gilbert
