problem with clipping

I recently got a new 2008 Honda Civic Coupe and I decided to upgrade the audio system in the car. I wanted lots of bass so I decided on buying competition stuff. I bought two dd2512 subs and a ddm1a amp. The subs each have 750rms and the amp I believe has 1200rms at 1ohm. I was told by many audio installers and other people that this should be really loud, loud enough to wear it would hurt to be in the car. So I got everything install into the car and wasn't too satisfy with the results. The car is not that loud (or at least as loud as I've been told it can be) unless I go into clipping, and I don't want to be clipping the subs too much since this could cause damage to subs. So my question is what can I do to lower the clipping in my car so I can play my subs louder and cleaner. Should I buy a line driver, add another amp, replace the alternator, add an after market radio, or anything else that I can do (including small stuff that will help)

In the car:
factory radio (with a RCA converter)
2 dd2512 subs in a ported box (the box is custom and all dimensions   
             are correct)
1 ddm1a amp
extra battery (dry cell, 850 creaking amps)
wiring gauge at 1

Also to help the clipping I have the radio at -6 (the lowest it can go) and the gain on the amp is low (about half)

Any info or help is greatly appreciated.
Thank You

what frequency is the box tuned to? even though you used the manufacturers specs they might not be optimal in your car's acoustic environment. so you could try to tweak the box's dimensions and port tunning frequency.

get a new radio, stock radio with converters is far from what you want. a decent radio with high voltage preouts can go a long way. also since you are using converters, where did you tap them at. some cars have certain speakers on a highpass filter so they wont get low bass frequencies, so people dont blow their speakers and so they sound clearer. not saying this is a fact for your car, but it happens, so usually the rear speakers are the best place to grab a signal for bass if you have to use converters.

also what do you mean when you say clipping, because if you have most of your settings mostly down you shouldn't be having any clipping issues. do your converters have a level adjustment on them? if they do you should turn them down if possible and turn the amp up a notch.

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What are the specs on the enclosure??

Also your subs are suited MUCH more to sealed, 4th order bandpass alignment. In a ported alignment the sub is quite peaky and rolls off very early.

READ HERE ABOUT THE 2512

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Custom Enclosure Design