I would add a second set of surrounds, place one to the sides of the back seats, and one to the sides of the front seats. Does the avr have preouts? If not, you could either wire two 8ohm speakers in parallel, which may be a heavy load for the receiver depending on the impedance curve of the speakers, ir wire them in series. Having surrounds in front of the back seats would really mess up the sound stage, especially if you're playing back something that's only 5.1 and not using matrixing to derive rear channels. Bipoles might work, but it's still going to be obvious the sound is originating from in front of the rear seats. Theaters have multiple surrounds for this very reason. You might also need two subs. In my experience placing subs in a room with open spaces requires the subs be near an area with three boundaries otherwise there will be a lot of cancellation and notches in the response. I would place them on the left side of the room with in the front and back corners, you might have to experiment with placement.
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