yeah i have the exact same speakers and problem you have. I was thinking of getting a single JL 10w3 in a sealed box with a coresponding amp to power it. my friend with a GTI has this and his system sounds great. i think he has a fostgate amp.

 

most HU's have a dedicated subwoofer output that you can turn off in the options menu in the deck. i know my girlfriends lower mid-end JVC has it, so i would assume that yours does too, but not positive. i would run a dedicated amp for your "bumps" and a seperate 4-channel for your speakers. you need all four preouts for front-rear speakers going into your speaker amp(s). you can just use the rear outputs for your sub amp, and that would allow you to turn off the subs, just fade all the way to the front. a lot of amps also have a remote gain control, so you can pretty much turn off the bass.

 

the front and rear preouts are not outputting the same signal, no single rca out of the four you have is emitting the same signal. they emit right rear, left rear, right front, and left front, depending on what rca is which,so if you fade or balance, it wont work, and if you have a song that starts out with like a drum solo from one side,then guitar on the other, then they merge, and you arent using the left side preout, then you wont hear whats happening on the left. same with rear/front.

the eq will work, it wont be the best way to go about things though. im not bashing your deck, you can make it work, but to have a full soundstage and really get the best out of your music, you need a deck with a non-fading subwoofer pre-out. that way you can get just what is supposed to be delivered to all of your speakers.

 

the sub should go on the rear. and i am not good ata explaining things so maybe this is better, the front rear action happens the same way the left to right does.

 

set bass on your deck at like -3 or something in the middle of 0 and the lowest point(usually -7), then you can adjust your gain on your amp to the loudness you want them at normal volume. when you want more bass, just turn up the bass on your deck, while giving more bass to your speakers, they will still be at normal bass when you are at 0, but your subs will be much louder.

it depends on what kind of crossover you have, if you can put in the rears as inputs then run off the crossover on highpass to the rear speakers, then off the lowpass to your amp, that would be best, most crossovers have adjustable points, and if you adjusted them the way you want, you wont get any bass to the speakers, it would all go out the low pass side to the amp and subs

 

sounds like a plan that will work

 

You can pick up a switch cheap at Radio Shack to cut the power to your switched power input. If you make a little hole in the console you have an easy way to shut off the amp.