Starting to lean towards two Ep 500's and hope I dont get the upgrade bug. EFalardeau how big is your room?
When discussing subs good posts to look for are from Jakeman or Craigsub who do extensive testing and auditioning.
I couldn't find a certain post that listed the combinations that Jakeman considered good to better but I did find another that covers some of your questions.
Post on dual EP subs I believe Jakeman has mentioned a couple times that he preferred 2 EP500's over a single EP600 though. Good luck and hopefully one of them will chime in.