pirate252 offered excellent suggestions, I agree with what he said. You get a better sound when the speakers face the rear of the car. I think dynamat is over rated though. All that stuff is glorified tar paper. I have gotten just as good of results with tar paper from a hardware store as well as the spray on automotive undercoating. Also check Ebay for some generic alternatives. If dynamat's name and claims give you piece of mind, pay the extra money for it.

I really hope you're not running that 800 watt amp off the stock alternator!