New sub or new speakers?

[tilapiah6]

I have a bit of a dilema. I have 500 dollars that I can either use to get speakers or a second subwoofer. You can see my HT photos on the left. Please let me know your thoughts on which you would choose. Would I benefit more from getting another A2-300, or upgrading my rear speakers? Towers, unfortunately, are out of the question due to room size, but I may consider bipolar bookshelf speakers for surrounds and surround rears.

As a last choice, I could get a bigger eD sub for a second sub, but I'd rather stay with the A2-300. Would my setup sound even better with a bigger sub added, seeing as that bass is non directional, OR would this cause the bass to sound uneven?