Isaac asked: “I have a Pioneer 5.1 receiver. A relatively new one, a VSX-823. I have 9 speakers hooked up to it with different Ohm impedances. I didn’t know if having more speakers than it was made for, or having them at different impedances, could damage it.”
Isaac, yes it could damage it. This is the easiest way to visualize speakers: think of a pipe with a steady flow of water running through it. The pipe is your cable, the water is your sound. The lower the impedance, the larger the pipe is.
If you exceed the number of speakers the amp is made for and you don’t wire it right, you are basically forcing a huge flow of water through a small pipe, and you can cause damage. In this case what could break would be your amp, as it would be forced to push more Ohms than it can.
Now, if you wire things right, you can hook up extra speakers, the trick is that the connection must be in series not parallel. You can see a diagram explaining how to do it here.
One thing to consider is that you may not get better sound by plugging in more speakers, so you may be better off testing the sound without the extra speakers. If the sound quality is similar, which it will probably be, you’ll probably be better off just using the amp as it was meant to be used and not risking any of your gear.