The concept that 10-inch subwoofers hit harder than 12-inch subwoofers is true. Having said that, I use crossovers and have the 10-inch subwoofers put out about 120Hz with a 12dB octave. The purpose of this is so all I hear is the punch bass within the music, like that from a bass drum.
I listen to R&B and old school by the way. Crossovers are also used on the 12-inch subwoofers, which are putting out 60Hz and below with a 12db octave. This is for the deep bass, like what you hear from a bass guitar.
When I designed this sound system 11 years ago, I wasn't going for how loud and how far I can be heard. My goal was
precision! I wanted to hear every musical instrumental within the song the way the artist originally recorded. I wanted 100% crystal clear precise music. I've accomplished that with this setup.
I actually have the 4 subwoofers working together without any issues or conflicts. With an equalizer and 3 crossovers, I was able to tune the system to the point where I can play any type of music and it will sound crystal clear without distortion or loss of sound quality. Rock, rap, country, R&B, opera, etc. You name it, I can play it without any further adjustments to the system. It took me months (if not years) to tune the system to this point. But I got it there.
As a side note, counting the 4 subwoofers, I have a total of 10 speakers in the sound system. All of them are powered by 3 amplifiers. I don't think that's overkill at all. It does the job that it's designed to do.
I don't get the point of your overkill here. Why have 10" free-air subs mounted to the deck if your going to have 12" subs essentially inside of the free-air enclosure (The trunk) They are just going to work against eachother and make the free-airs sound out of phase and pointless.