J2, welcome. The fact that the 609, as well as most other receivers doesn't carry an official 4 ohm rating shouldn't be a matter of concern. To have such a rating under FTC(Federal Trade Commission)amplifier power regulations, they'd have to drive a 4 ohm laboratory test load at full power for at least 5 continuous minutes. This sort of thing doesn't take place in the real world of home listening and typical receivers such as your 609 can do fine with the M80s unless they're played at extremely loud levels for a long period. The M80s are slightly above average in sensitivity(therefore require less power)and average about 6 ohms in impedance over their frequency range.
