I'll post the same response I made to syxx elswhere on the boards about his Kenwood and driving a 4-ohm load, because it also applies to Onkyo:
"Nor will I tell you to ignore the 4-ohm impedance of your M80's. Look at the back panel of your Kenwood near the speaker binding posts. What does it say? Is there a switch you can move "For speakers of less than 6 ohms impedance" or it might say "For low-impedance loads--4 ohms".
It might also say "do not use speakers of less than 8 ohms (or perhaps 6 ohms)", in which case, be careful. The M80s are very sensitive, which means they'll play very loud on only a few watts of input power, but their impedance curve touches 4 ohms in a couple of places. That means lots of current will flow through your Kenwood's output transistors. Put your hand over the top of the receiver: it should never get hot (warm is okay) and be sure it's well ventilated.
If there's a switch for 4-ohm loads, that will impose some current limiting on the output stage, which is okay as long as you don't try and overdrive the Kenwood at high levels. If you do, it may just shut down until it cools, or you reduce the level. It's a protection mechanism so you don't blow your output stage."
All this applies to the Onkyo. They offer a lot of features for the money but tend to skimp on the heat sinking and output stage cooling, so that limits the Onkyo's potential (usually) with low-impedance loads. You'd be better off with a heavier (literally) receiver like a Denon, which is stable with 4-ohm loads and has lots of heavy heat-sinking to keep those output devices cool.
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