Q:
I would like to run an extra set of sub speakers on the same amplifier output but the speaker impedances are different, will this work? 

A:
No, when the PA-CP amplifiers are set to the 8ohm constant power setting, each speakON output is designed to be connected directly to a single 8ohm passive speaker. 
When the PA-CP amplifiers are set to the 4ohm constant power setting, each speakON output is designed to be connected either directly to a single 4ohm passive speaker or to 2 x 8ohm  passive speakers.

A workaround is to run the 2 x 8ohm PA speakers from one speakON output and run a single 4 ohm sub speaker from the other speakON output although this will mean that the speakers are running in a Mono configuration.

To run 2 x 4ohm passive subs and 2 x 8ohm passive speakers in a stereo configuration an additional amplifier will be required.

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