Hi jhunt,
Your M60s are 8 ohms so the Yamaha should be quite comfortable driving them cleanly without breaking a sweat in that size of room. And you're fine at 75 dB SPL. It's also a matter of duration and giving your ears a rest.
Long-term hearing damage starts to occur if you are subjected to levels of 85 dB SPL for 8 hours a day or more. That's also the rationale for THX specifying cinema playback levels averaging 85 dB SPL in the middle of the auditorium. It's quite loud, but it's only for a couple of hours, so you don't get long-term exposure.
I would add, however, that some THX movie theaters greatly exceed that specification, notably the Ziegfeld theater in Manhattan. I attended a press screening of a Star Wars movie (the third one, I think) where the levels were well above 100 dB SPL. I left after 30 minutes. It was deafeningly loud. The screening was sponsored by a big electronics company and I think they suggested to the cinema operator to crank the levels, which was stupid.
You can safely run your system to occasional peaks of 100 dB SPL without danger, so long as you give your ears a rest after the movie.
