Guy,
Do not let him run the amp off the speaker cables. I guaranty you will have poor sound quality. That is definitely a jerry rig.
You need to have a set of pre amps to run a new amp properly. I dont know if the stock amp and/or head unit has preamps. A lot of car manufacturers wont put preamps on their systems to discourage aftermarket stereo work.
By the way, I love that new verb! "sub-woofing"
Andy,
What do you use for pre amps to your new amp? Do you have the stock amplification powering the MBQuarts? Is it enough power for such a high end speaker?
I bet your subs are LOUD! Are they boomy in the free air setup or are they pretty tight? Did you use any dyna mat or other vibration absorption to avoid body panel vibration?