i had a 18 in a 4th order tuned to 50 hz and i played everything from 28hz to 65hz with no prob. also got loud with only 1500watts going to it. never struggled or anything while still having sound quality. not saying that a 4th order is better for daily than a ported just dont want you to go on thinking wrong things about a 4th order
well, considering that I built my first 4th order while you were still learning to walk, I'm not to concerned about 'thinking wrong things about a 4th order'.
Bandpass boxes have all the downsides of a ported box x2 (peaky response, chuffling), but you get to trade some space back for the ability to play (at reduced output) below the tuning freq, or compensate for really really bad choice of drivers.
better to build a low tuned ported box, or sealed if you want to do anything other than play boom boom one note wonder 45hz Master P/ 3rd coast rap, or dance/electronica.
For those that only listen to that music, it is a viable choice, though most everything else (and even some of the former) will sound like doo-doo.
They have their place, like a box that must be ported into the vehicle cabin from the bed, or people who want maximum output in a very limited bandwidth and don't care about non-linear response... ie SPL freaks and teenie bopper SPL wannabes.
Last one I built was a T design with two 12s and had a two piece telescoping port to help tune in-vehicle response ~35hz. it Played most effectively from about 25-45, but even on some of my 'street' music, the lowest bass notes were noticeably subdued to the point of feeling absent in comparison.
