What he said.
I'd strongly suggest you to listen to a sound system based on MS-8 configuration with central speaker.
If you look on the UK forum, Rudz has a sound system similar to what you are thinking of doing (except his under seats are SWS-8s).
If you want Jenherts, then you'd require a sub.
Alternate is to have SWS-8s now and change them to Jenherts when you add sub.