Hi medic8r,
Based on our past Axiom customer experiences (and my own as well), Arcam receivers will drive 4-ohm M80s with no problems. As to Marantz and Onkyo, the record is spotty. Many Onkyos shut down or overheat with 4-ohm loads, and some Marantz have also not coped well with 4-ohm loads.
Since the Marantz in the store drove your M80s (did you run it fairly loud for an extended period?) you should be OK, but one of the reasons Axiom likes the Denon, H/K and Outlaw brands is that even the less expensive receivers from those brands will drive the M80s without overheating or shut down.
Regards,
