You would have to ask Axiom, it is their forum and they make the rules.
Many people run a Phantom center speaker, and it works very well if your in the center seat, however, as soon as you move to the left or right seat the illusion of the center collapses and does not work as well.
You don't tell us what receiver you have, however, AVRs don't care if a speaker is 8, 6, or even 4ohms, or any combination of these running in the same setup. Also, you want to leave your receiver switch on 8ohms, even if you have other speakers that are 6ohms in your setup, this reduces the voltage your receiver can supply.
