That Onkyo should have no trouble at reasonable listening levels (even some unreasonable ones) driving the M80s. Especially with a sub, as the reason of lowest impedance is in the very low bass range.
You'll have a hard time finding a receiver that actually advertises 4 Ohm stability. Most will say 4 Ohm is supported if the current limiter circuit is enabled, which compromises performance. Yet, at the same time, most receivers actually don't have a problem with 4 Ohms unless pushed to maximum volume levels. Additionally, they have safety shutdown features that'll turn the receiver off if it does get to hot. So if you shut it down, don't play it that loud for that long again (your ears will thank you too).
