Increase in sound with more watts?

A bad marriage.  Divorce is imminent. 

The L5 is a SPL sub that likes a lot of power and a big enclosure.  You have neither of these, so you should consider a high efficiency subwoofer that doesn't mind a small box.  And you should also consider using that amp to power a set of speakers and buying a more powerful mono amp for subwoofer use.

Find out what amp you will use first, then pick a sub based on the above criteria PLUS its impedance (which means you'll have to find out what the coil(s) is rated at).

Not trying to slam you or anything but that is not correct.  A "vented" enclosure does not make a speaker act like it's in a larger enclosure.  Bass Reflex (vented or ported) enclosures simply function differently from sealed ones, with a 3-6db bump in output at the port tuning frequency and a loss of enclosure-related speaker damping below the port tuning frequency.  Vented enclosures act much differently and must be used differently (such as adding subsonic filters) from sealed enclosures.  And mounting a sub in a tight space may or may not have any effect whatsoever on output, as long as the woofer can move unimpeded to its excursion limits and the port (if used) is not blocked, the air space clearance required is very small indeed.