Mixing Speaker Impedance?

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.

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.

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