Matt, welcome. The reason that the 809(along with most other receivers)doesn't have an "official" power rating at 4 ohms is that under the FTC power amplifier regulations it would have to operate at the full rated power into a 4 ohm test load for at least five continuous minutes. Few are designed to do that without overheating and shutting down. Outside the testing lab in the real world of home listening these receivers nevertheless put out substantial power into 4 ohms during times in music when the load happens to be at that number, and no problems occur.
There doesn't appear to be a published lab test on the 809, but similar units show about 50% higher numbers at 4 ohms for the brief periods that actually occur in music. These units are being used every day for speakers similar to or more difficult to power than the M80(it's slightly above average in sensitivity and requires a little less power). There's no good reason to expect that the 809 wouldn't be fine with the M80s at all safe home listening levels.
