Hi dapman,
I'm delighted you like your new M80s and it's no surprise that your aging H/K drives the 4-ohm impedance. While I appreciate JohnK's frequent and knowledgable contributions to the Axiom site, I should tell you that most Onkyo AV receivers go into severe current limiting when connected to the 4-ohm M80s; indeed, many shut down.
Another AV receiver option to consider is Sherwood Newcastle. Axiom now has a relationship with Sherwood and we've bench-tested most of their models at our lab. Even the entry-level AV receivers drive the 4-ohm M80s with no current limiting and lots of output. Look at the Sherwood R-671.
