If I connect two pairs of speakers but use EITHER speaker output A or B (nor A+B) is there any change to impedance?

If using either Speaker A or Speaker B outputs, (not Speaker A and B at the same time) then this will not make any difference to impedance/power output.

If using Both Speaker A and B at the same time the following applies:

When you connect two pairs of speakers to an amplifier, the combined impedance is halved. So, when two pairs of 8Ω (Ohm) speakers are
connected, the overall impedance becomes 4Ω, even though the individual impedance for each speaker remains unchanged at 8Ω.

**
Can I use 4 or 6 Ohm speakers with the AXR85/100?**

You can use a pair of 4 or 6-ohm speakers, but we do not recommend using two pairs of 4 or 6-ohm speakers with the AXR as this could result in the
amplifier overheating, which in the long run could damage the unit.