Mixing 4 and 8 ohm speakers

I drive a pair of 4 ohm KEF Reference 107/2s with a Proceed HPA-3 amp (500 watts/channel into 4 ohms, 250 into 8 ohms). I'll be adding an 8 ohm KEF R 600C center speaker. The HPA-3 is essentially 3 amps, each with its own power supply, that share a chassis. Need I have concern about mixing output impedances? Of course, I'll match levels with an SPL meter.

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As long as you can adjust the gain individually for each channel with you processor, you should have no problems. You still have to make sure the amp is up to the task of driving all the speakers, but you have to do that with every system. Your HPA-3 should have no problems at all.
The center channel is a separate amp. You are not mixing the impedance with the main speakers. You won't have any problem.