Mixing Speaker Impedance?

If there's only one switch than select for the lowest impedance.

Wont setting the amp to 4 ohms limit speakers with 8 ohm requirements?

It will limit amplifier output characteristics in all cases you set, but if done to 8 ohms, your amp may half or even quarter the power to the 4 ohms while to 8 ohms it may have some voltage deficit and probably just half the power.
Last thing you want to do is to half power to your mains. your center channel will mostly work on mid-range and will have enough spl volume.

"08-28-13: Azag
Wont setting the amp to 4 ohms limit speakers with 8 ohm requirements?"

It shouldn't because you will be setting the volume levels for each speaker when you do the setup with your preamp. The only other question is to weather the amp has enough power do drive the speakers. MA is not a difficult speaker to drive. I can't imagine either of your 2 choices not working.

The switch is for the amps protection so that it doesn’t get too hot.

Wont setting the amp to 4 ohms limit speakers with 8 ohm requirements?

This setting limits the amp, since it alters the operation of the amp to run cooler, that typically results in less power output capability. The 4 ohm speaker requires the most power, with the 6 ohm being less, and the 8 ohm less than that. Also, the size room and how loud you play the system are factors. Obviously, louder volume and larger rooms require more power. In a way this may not make sense, but as I said, it is to protect the amp, by limiting its output to stay cooler. Ideally it is best use a 4 ohm rated amp with 4 ohm speakers.

Just curious as to the choice of a 9 channel amp for 5 speakers, if you don’t need the extra channels, why not get a 4 ohm rated 5 channel amp?

If you have extra channels, try doing a vertical biamp. It may sound a lot better.

I believe it should be EVEN number of extra channels for vertical biamp.

Sorry Zd, a vertical bi-amp means using one stereo amp per speaker, therefore L & R are totally independent. Horizontal bi-amp is a one amp for low input of both speakers, and another amp for the high input of both speakers. We will probably need to call this scenario a horizontal bi-amp since all channels are in the same chassis.

The amps rated power is 150wpc with 2 channels driven, and HT Review’s measured power on the amp is 119wpc with 5 channels driven. The OP said he was considering the purchase of this amp, so IMHO, I would take that amount, plus the additional for extra interconnects and cables needed to bi-amp, and purchase a good 4 ohm rated 5 channel amp with more power. The RX8’s will perform so much better, and the center and surrounds will benefit also, especially, if he is in a large room.