Axiom does not work through dealers, they only deal directly to the customers. This cuts out the middle man and you get the savings, so not sure if someone is trying to pull the wool over your eyes or not.
2 identical subs placed correctly in a room, will help to give you a better freq response for all seating locations. Two 500's have ton's of output, and 2 800's would give you more. The question is how big is your room? This will help determine what sub you might need.
Not sure I follow your last question.