Scott,
Typically consumer speakers are rated at 8 Ohm, most portable professional speakers at 4 Ohm, and stationary pro speakers (like in the concert halls) at 2 Ohm.
This is how 8 Ohm/4 Ohm rating is applicable, in the case of PMX2000:
If you plug in one set of speakers, rated 8 ohm, you get 180 watt sound output. If you plug in a second set of speakers (in parallel with the first one - some amps provide dual outputs), this halves the impedance to 4 ohm, but you do not get twice the power output, but only 250 Watt.
It follows that you have to match the impedance of speakers to the amp - for maximum output you should get 4 ohm speakers, or use 2 sets of 8 Ohm speakers. I would not recommend getting speakers with lower impedance than the amp is rated for (possible to damage the amp).
BTW, despite some posts to the contrary, I still believe that PMX2000 is a MONO mixer, as you can not pan the individual inputs to the right and left. If you think it sufficient for your needs - fine, but I would spend another $70 and buy a true stereo device.
Regards
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Regards,
Alex