You may want to consider running the 4 co-axial speakers off the head unit and use the 2- channel amp for your sub. If you put the sub behind your observer's seat, you can get away with a cheap enclosure.
You can spend a little more and get an Audioformz fiberglass enclosure and an inexpensive sub, and you will have a durable, inexpensive, expandable, high qualty setup.
Check out the 2002 closeouts at
www.audioformz.com.
Great stuff!