I think the general def for SQ is if ur system has the ability to play all frequencies at a flat level. Hence the reason so much turning is required. Of course people like different sounds and so they adjust it to however they like it. When u tune higher it's louder because it peaks higher toward those frequencies u tune to. 35-45hz is usually where most bass notes in rap/hip-hop songs hit. But when a sub woofer has to play below the box's tuning point it has a higher chance of over excursion, because there is very little to none dampening (don't know if thats the correct term) below the box's tuning frequency. That's why there's subsonic filter on amplifiers, to reduce the output below the tuning frequency of the box. People tune lower (28-30hz) to try to get the flattest response in all frequencies, That's the main reason they say to tune low for SQ.
That's the quickest way i can sum it up. Of course if u really want to get into it, it gets alot more complicated. And u can find response graphs and stuff to help prove the point. 32-35hz seems like the midpoint to be able to get loud, and still reproduce the lower frequencies well.