I think the Denon should be able to handle 6 ohms. If the choice is between the EMPs and the Aperions, I would go with the EMPs. I think the introductory price is worth taking advantage of, and may be a better value overall. Of course, which is better sounding to your ears is the best deal in the end.
I would add that the EMP sub looks a little weak, at least for my tastes, but I am something of a bass-head. It looks nice, and if that is what is important to you, it may not be bad, but a 35 hertz extension? That is pretty weak for a sub intended for home theater. In that same price range I would rather get something from BIC Acoustech, who probably make the best subs in the $300 range, at least for home theater. Look at the PL-200, or the cinema series H100, or the BIC America VK-12. If you can spend more than that on subs though, you really should. I would be looking at something from Hsu, SVS, Outlaw Audio, or Elemental Designs.