Additional amp/sub with Logic7 sound system

[Pit Z3]

Hi,
i´m thinking about to install an additional subwoofer to my Logic7 system in a 2002 E65 745i... but up to now, i have not an good idea, where to get the signal for the additional amp ...
Has someone already made this ?
The easiest way would be to install a High/Low adapter - but i´m not sure, if all speakers get the "full signal" from the amp ( especially the lowest frequencies ) ...
It would be great, if someone is able to give me any informations, which would be helpful for my "little project" .

Regards Peter