Well here it goes.

Putting a cap on the speaker to filter out the low frequency information is not going to work all that well. Internally the amplifier and the DSP that drives it has a variable loudness curve which is tied to the speed of the car and the volume setting. Thus in order to get some additional volume from the system you can reduce the bass level on the radio, -3 should result in you taking the volume up to (30) with no issues with a strong recording. Given the bass control is centered at 100 hz this reduction is taken into those calculations.

With the factory amplifier/speaker combo running at 2 ohms you are getting 40 wpc to the doors. To get 3 db more (slightly louder) is going to take twice the power or 80 wpc, 6 db from stock 160, all assuming the factory speaker can take it.

The first step would be to change out the speakers for a 2 ohm set that is more efficient. Not too many of those on the market (Infinity Kappa is one of the few). Assuming the factory speakers are reasonably efficient say 90 db, the move to Kappa 60.7 components would yield 5 more db of output running off the factory amp. You will still retain all the eq, compression, limiters, and speed sensitive volume of the factory system.

If your needs fall to the lower octaves then there are solutions like Infinity basslink that don't take up too much space. This leads me to the answer to your question about 12v going to the sub, well it is either a misprint (some older Infinity systems located the amplifier on the speaker and thus needed a line for power and one pair for signal). I believe in this application what you are seeing is the use of the additional 4 channels from the amp paired up with each sub. In essence the system delivers 240 watts (40 to each door, and 80 to each sub).

Frankly while this car would be easy to retrofit, the space considerations are significant. The trunk has a flat floor with two small pockets but their location is not conducive to building some type of custom enclosure. You could fabricate something in the wheel well, but again your won't gain much. Generally the subwoofers on the market require .75 cft for a small 10" sub enclosure (Kappa Perfect VQ solves this issue - only .3 cft per if you are willing to give up some output around 20 hz (not an issue in this car due to the hatch providing gain), so the question is how much space can you afford to loose, AND, oh year remember this car has a low load capacity to consider, so keeping the weight down is critical unless you plan on doing some serious suspension work.

I have been building my SRT6 over the last year (I travel quite a bit so it is now in phase two) with the approach of integrating everything with the factory system. When it is done it will be a simple two woofer, two amp, one component set system with a HK Drive+Play IPOD controller. No video, no cheap looking graphics or "look at me" paint jobs, just a great sounding system that speaks loudly about craftsmanship and class. The dealer I am working with is www.imagineaudio.com, and so far they have met my criteria for excellence.

System details: Factory Nav with CD changer, Harman Kardon Drive + Play, Infinity Kappa 60.7 CS, (2) Infinity Kappa Perfect 10 VQ, Infinity Reference 1210a, Infinity Reference 7520a.