question on 6cf box for 15 L5

First of all, you are robbing yourself of the lowest bass frequencies.  Set your subsonic filter to a much lower value, like 10 Hz.  The Solobaric 15" L5 will respond down to at least 18 Hz.

Second, Kicker's own technical information says the 15" in a 6.0 cu ft vented anclosure has it's peak output at about 95 Hz, so by crossing over at 80 you are robbing yourself of peak SPL.

Third, there is no reason why you cannot design an enclosure with the vent on the front baffle.  Just make sure you do it correctly.  The technical manual for your speaker shows a sample of this sort of enclosure.  If you don't have the tech manual, it is available on the kicker web site.

Cheers!

Ah, then that really is not a "sub" sonic filter, eh?  I know many companies call them that when they are actually high-pass filters in the "sonic" range!  A true sub-sonic filter is below 20 Hz.  Mine is set at 5 Hz just to keep any stray DC out of the system.

Try by-passing the subsonic altogether and see if your deep bass improves.  The only real advantages you will get with an L5 moving to a larger enclosure are higher SPL at a slightly lower volume (response peak moves from about 95 Hz down to about 88 Hz) and a potential for deeper rumble in the 17 to 22 Hz range.  So your sub sonic filter is removing all of the gains you might feel (you really can't hear them) in the very low frequencies.

The best advice I can really give you in building a large enclosure is bracing, bracing and more bracing.  You want that box to be VERY rigid.  I have a 16 cu ft enclosure for two 18" North Creek Leviathan subs in my home theater, and it is lined with concrete just to keep the thing rigid... RTA says 92 db at 10 Hz, and this with only 250 watts!  It shakes the foundations of the neighbor's house!  I've always wanted to try one of those babies in a car, just for grins...