Changing enclosure volume for SPL?

Im bout to build 2 ported boxes for my friend. He has a 97 dodge single cab, and listens to nuthin but rap and wants it loud. right now he has a sony xm-2200gtx which will push 250 x 2 rms @ 2 ohms or 500 x1 @ 4 ohms bridged. subs are 2 sony XS-L121P5. i told him to stay away from sony but he just cant do it so avoid telling me to to tell him to sell it all. well sony recommends 1.58' ^3 with a 3" x 8" port, but there is plenty of more room behind the seat for more room. i was wondering if making the box any bigger would yield more spl with a decent response curve and not a big spike on a 10htz range. ive been messing around with winISD pro but im not exactly sure what im doing.

heres a good article about the sub with specs and all

https://carsound.com/articles/publish/printer_17.shtml

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Sony's recommendation looks like (on WinISD) it creates a huge spike in the 40 - 50 Hz range.  The port is too small, but that is commonly seen in recommendations from the maker because longer/larger ports aren't easy to come by off the shelf.  The volume is a little small, too.  A good workable enclosure for each sub looks like:  2.0 ft^3....3 X 12 port  (tuned at 26 Hz).  But keep in mind some individuals like the results of bass boost, too....which is akin to the huge spike Sony has provided in the specs!  He might get the impression of louder sound if you just use Sony's data with the spike.  What I gave you is a flat response for higher quality sound.  I believe in power to provide the SPL, not manipulation of the response curve.  He should be thinking about trading up in amplifiers for these two subs for rap.

another question or 2. (by the way thanx stevdart). since his amp doesnt have a subsonic filter, would the best option be for a 30htz high pass f-mod? and because the truck is a single cab, i can't make the box  very deep which would elimintate the option for a long port right? or would i be able to bend it or sumthing? This is my first ported box project.  

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what about 2 ft^3 with 3 x10 port @ 29hz? cuz this kid doesnt really care much bout sq, he'd rather be a blockrocker

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Using the f-mods would be the smart way to go.  Yes you can put a bend in the port or even allow it to open on the side to make it fit.  The size you listed should work out fine.  With the tuning freq below the subsonic filter cutoff, you won't have problems with losing control of the cones below tuning frequency.  So make sure you use the 30 Hz f-mods.