Honestly I am not sure what a comfortable listening level is for me decibel wise. I can say that the volume level I like listening to it at, it can get much louder without distorting. If you couldn't tell, I am not knowledgeable in the different terminologies.
A free decibel app for your smartphone can give you a general estimate of your usual sound level.
I have roughly the same equipment as you, with the M5HP and an Onkyo TX-SR393. I sit about 8 feet away (see the gallery forum for my setup), and have Dolby Digital enabled (which is ~3dB louder than stereo or stereo direct mode). I usually have the volume between 35-46. If I'm listening to music and I want it loud, I set it to 52 or 53, which corresponds to around 65dB at my seating position, average weighted. I might hit temporary peaks of 77dB at that setting.
I have the AVR's max volume limiter set to 65 so I never bother my neighbor who shares a wall with me. I've tried level 75 when the neighbor's away, and the couch is shaking and I can't even see straight, it's so loud.
