its not how you build the box with the L7s if the box is vented you get huge bass and if its sealed you get a little sq with a lot of spl too but with the 15" L7 you need at least a 6.0 cu ft vented box for 2 subs and up to a 12 cu ft box so you need to have a lot of room to put it in, and it wont fit in a normal car.
Well it does matter how you build the box when you consider flex. Flex can hurt your spl. Granted sometimes in rare occasions it actually helps. But most of the time flex will hurt your SPL score. So if your building a large enclosure bracing becomes a big part of how it sounds, and what kind of numbers you put up.
but to answer the question
SPL does equal vented in one way shape or form //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif