Does anyone know if the Onkyo TX 8511 can handle the impedence of the M80s?

your best bet is to contact Onkyo and find out, or read the owners manual and see if it can handle a 4 ohm load.

Do you already own the M80's? If not, and your Onkyo can only handle 8 ohms, then the M60's aren't exactly a compromise. The difference between the two is the M80 can play louder, and is brighter and more forward. It's not a secret that many, many people prefer the sound of the M60.