I was running an inexpensive 8" JBL sub with my Axiom sub. I think with a large sub like the 500, a sub of a much lesser quality will not have much of an overal impact.. At least this is my experience the JBL made almost no audible... I would tell you, use the Polk for now, It's already bought and paid for :), that's what i did until i bought my second Axiom sub. When you get everything up and running you can make the decision if you think the Polk will be sufficient, or if it would be worth it to you to get a second 500... I think Dual 500's will have more output than the 500/Polk set up, especially if the Polk is a small sub.
From my experience when running two subs placement becomes less of an issue.
I put my subs where i wanted them,and i think my room sounds pretty darn good, with the power and extension of a pair of subs like the 500's i believe you will be able to do the same, with a similar result.
Good luck in your endeavor.
P.S.
You may want to watch this video on multiple subs, you may recognize some of the people in it..
http://www.youtube.com/watch?v=ayOXz26qm3I
