JL Audio Cleansweep installed into the trunk, spliced into the factory speaker wires. Then RCA wires from the cleansweep to two amplifiers mounted in the trunk also.One amp drives the front end which is Bostonacoustics Pro60 components in the front doors, Polk Momo Carbo 6.5s coaxial mounts for the rear doors with external crossovers.
The other amp drives One JBL GTi directdrive 12" sub with about 600wRMS in a .75 ft^3 box. This woofer is outstanding for the money. THey used to be pricey but now they are more affordable and it just hammers out the bass. It is a bit heavy, about 35lbs though.
Anyway, you cut the speaker harness when it is on its way to the door speakers, route the signal side into the JL unit. The output is RCA's, four or two channel and go into the amplifier. THe DSP in the JL unit calibrates the radio output so it produces a completely flat output signal to the amps in spite of any DSP action the head unit does on its own. THe only negative is you can only calibrate the volume on the radio at one position because if you cange the volume you "may" cause the internal DSP in the head unit to alter the radio signal more (as in reduce bass when you turn up the vol to prevent distortion in factor speakers) and in so doing it will throw off the calibration until you return to that volume setting.
So the JL unit provides you with a remote volume/switch knob that I mounted next to the bezel where the 12v adapter is aft of the shifter in my car. It came with two knobs, one in black and one in brushed aluminum just like the center console stack in the volvo so it matches very well.
Anyway, the volume knob is also a push button that changes over to remote Aux input for the cleansweep.
So I ran the rca-stereo jack cable to the back of the console stack where I can now plug in my MP3 player/Ipod and get any kind of audio signal right into the JL unit without any distortion or secondary devices....and hence a clean signal right to the amps.
Perfectly clean sound and costs about the same as if you upgraded your car in the old days with a new cd player, speaker and amps. I did all the work myself and spent about $1000 on the speakers and JL Cleansweep.
I already had the amps so that was a pretty good savings.
Still, I bought all pretty high-end equipment.
The JL costs about $350, a decent 4 channel amp is about $250, Speakers can be had for another $350 for component sets for all four doors.
So $1000 in parts if you do it yourself is reasonable for very good clean sound.
Figure $1500 if you pay someone to do it right.
For high end equipment and amps with a sub like I did and paying someone, figure about $2000-$2500 for what I put in myself.