*don't forget the batteries...this could account for 1-3 dB
*use an aftermarket high output alternator...try to get an adjustable regulator...this can often gain .1-1dB
*make sure you match the impedance of your amps to the impedance of your sub...it is safe to say that if you amp maxes at 1 ohm, you need a .5ohm DCR to get full power after considering for A/C inductance and impedance rise of the enclosure!
*Use a ported box...yes you can get loud with sealed, but ported gives you more bang for the buck
*don't bother with things such as 1/4 waves or 1/2 waves...they are just a waste of time...also, don't let computer programs dupe you into building the wrong box!
A couple of safe boxes...
for a 15...try 5.0 cu ft total with 65 square inches surface area port that is 16" long
for a 12...try 3.3 cu ft total with 50 square inches surface area port that is 15" long
*remember...it is power per driver, not the total number of drivers that will make you loud!
put your woofers towards the passenger side and your ports towards the driver side...
don't use caps...they will hurt you in SPL!
go for amps with unregulated power supplies...they are often better for spl purposes...(they tend to offer more power when given higher voltage input)
use a preamp whenever possible
do NOT match the gains on your amplifiers...this is bad...your best bet is to unhook your subs, play your peak frequency, crank volume to full comp setting...then test voltage on each amp...do not match them, but rather turn the gains till each amp PEAKS out...who cares if one does 65 volts and the next does 91 volts...power is power...the meter doesn't know which amp is putting out more power...
the list goes on and on, but that should be a good start
When in doubt, go with the woofer that sets the records!