I can't say that I have listened to the Infinity line-up; the big difference that would be noticed by going with an Emp Tek set up would be the inclusion of another sub. I hate to reference ratings when advising on a purchase, but the difference is quite substantial as the subwoofer from the RM6750 is rated down to 45hz, while the Emp Tek is good down to 30hz. I noticed a huge jump in performance making the switch between the Polk sub and the Emp Tek sub.
Having said all that, the Primus has a great reputation on these boards and elsewhere. If they would fit into your decor better, and you are either happy with the current sub/were willing to grab a separate sub from SVS via Sonic Boom or Axiom, I would go for the Primus as the Emp Tek require a certain kind of room before they don't stand out.
In comparison to the Polks, the only reason to go with the Polks is the size difference. The Polks will be a lot smaller than either option; if that was one of the major drawing points to them originally, then I would break out the tape measure before purchasing something new and confirming that whatever you end up with will still fit.
Regards,
Bill