Yeah. I would imagine you could figure a way to run a line level signal to an amp to power the subs... but for all the finagling and wiring it would take I'd say the all in one is the way to go.

 

sorry do dig up an old thread, but could you simply mount a new 8" sub in the factory location with a new amp and head unit?

 

anything is possible. but youd need to cut and fabricate. it will not fit direct replacement because 1. the hole is too small 2. there is not enough clearance for the woofer magnet.

 

The pictures are of what I had in my XJ40 4.5 coaxial"s fronts 6x9 coaxial's rear deck 15 inch sub in boot couple big amps had to really drive it with all the sound insulation and fuel tank in the in-between the tank and rear seat

when I bought a XJR6 the boot was to small I installed these bass engines Aura Bass Shaker
I have one under each seat and at least I can use the boot

The boot in the XJR6 is smaller that the XJ40 pretty happy with the sound of the bass engines and unlike the big sub-woofers its not rattling my car to pieces

I am still in the planning phase of the new X350 I bought as the sound system in it is a big step forward from the previous cars but still pretty mediocre

 

In case anyone is interested, here's a couple of pictures of the new sub box I've had built for my X308.
It took a lot more time and money than just putting a prefab box in there, but the end result is looking amazing.
You'll have to excuse the overkill on the bracing etc, it's designed to have a VERY heavy sub hung off it




if you click the pics it will take you to the whole album

 

You can get your rear sound supply from the tweeters in the parcel shelf ,just in case anybody needs to do this job