Tuning - It's usually trying to find a balance between max extension and max spl. In this case it would be 12.4 cubic feet tuned to 16 hz.Thanks Brent. So that begs the question - should I tune for max extension (below 20hz) or max spl (from 20hz)? Is there a THX standard for extension? Also, how important is it to get a flat curve for the transfer function magnitude & how does that curve relate to max SPL and which one is most important (I know, probably both). In fact, I've got no idea what to do with WinISD - e.g. the best tuning for my application - lost!
It's ok if the transfer function dips below 20hz because room gain will add to the output and bring it back up. Some rooms have more room gain then others so it's not very accurate. To get a flat response in room you would use the REW software to get an accurate response and use the BFD (Behringer Feedback Destroyer) to make the corrections.