Oh right. I forgot you're using the subs as stands.
If oomph is all you are worried about, stick with the 500v4. If you want low end extension to 15 Hz, go with the 600v4. I would bet however you would be just fine with the 500v4 and you'll achieve a good 18 Hz in your room.
A single 500 is adequate for 5,000 cubic feet BTW. If you got stands, you'd save the cost of the extra 500. The 500 doesn't have to be up front. My 800v4 is slightly behind me firing into the diagonal of the room. This allowed me to get the smoothest response.
You're gonna love the sealed 500! You will kick yourself for not getting it sooner.
I still advise buying the center, M5, QS10 and sub on a trade-in deal. You'll save a lot of money. If you can't afford it all at once, maybe Axiom can have you make a few payments. Feed your family canned tuna for a few months. It will be worth it.
