after looking at the specs your not going to get more out of those subs then what they are doing now...They have pretty low power handling around 100wrms and an spl of 88db....If you want louder it may be time to upgrade
my suggestion would be to not use the 5.25 due to cancellation problems. Also use the 6.5 as dedicated midbass from 90-250 and the 4" 250hz and up. using different size speakers playing the smae frequencies can also cause cancellation....If your just trying to get really loud upfront switch to just multiple sets of 6.5"
4 ohms is lower then 8 ohms so
which will be perfect for your amp if it is 4 ohm mono stable....
the easiest way to build your first enclosure is to follow the manufacturer recommendations for now. Then as you learn you will get more ideas on how to improve...or have a shop do it for you
looks like your gonna have about 5 ft3 of airspace. I would suggest two twelves and ported....As far as brands Fi Q's or Digital Designs 9500 is what I would suggest. Since you didn't list them here is the Digital Design's site http://www.ddaudio.com/dd/caraudio/woofers...sp?series=9500f
g/l
did you say Sonic Electronix was a good experience??? Their prices are just as good as e-bay....it would be nice to know of positive feedback....and if they suck I'll blame you
with two dual 4 ohm subs you can wire them down to a 1 ohm load at the amp = more power
with two dual 2 ohm subs it's either 2 ohm (less power from amp) or .5 ohm (which will send his amp into protect or blow it
this is a good read to understand a little better..... http://www.installer.com/tech/aiming.html
He doesn't seal off trunk and have subs in car ,but this does make it easier to undderstand why this works