DB’s/ car audio advise

I have a 2011 hyundai elantra
H/U: Kenwood kmm-bt3U
Amp:Digital Designs m80b
Subs:Svc Kenwood 12's @ 1.3 ohm
Box:4.5 cu/ft ported tuned to 25hz
(Big 3 upgraded to 2awg, stock battery and alternator, and power/grn wire to amp is 2 awg)

Now then, my question is, accourding to my DB meter with my windows up, doors shut, and 1 of the backseats down
@20hz -100.1db (spl)
@25hz -100.2db
@30hz - 100.2db
@35hz - 100.2db
@40hz - 100.2db
@45hz - 100.2db
@50hz - 100.4db
Amp has a clip indicator/remote/digital voltmeter, the amp isnt clipping when i play the test tones, shouldnt i be hitting louder than 100 db? I mean my dad has 4 orion xtr 12's, a type r 12 and a rockford p2 12 in a 7.9 cu/ft box ported tuned to 35 hz with 2 aftermarket batterys stock alternator ran off 0 gauge on a autotek meanmachine 3000.1 @ 1.3 ohm and my db meter says his only hits 100.5 db @40 hz it seems like hes hitting alot harder than my system but accourding to the db meter its only .1 db higher than my system what can I do to get higher DB in my elantra? And how can my dad get more DB in his stratus?

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Basshead97$