Peng
I don't understand how you can have a meaningful discussion about listening levels (either music or HT) by folks telling each other what their volume knob is set at. With all the variables of power, listening distance, speaker efficiency, source content etc etc, it seems the number on your volume control wouldn't mean much as a person to person comparison. Wouldn't SPL at the listening position be the only way to have a meaningful comparison point? You listed mid 70db as your choice. I assume this is mid 70's db SPL as measured by a meter.
To me, the SPL at the listening position would be only only way to have a decent shot at comparing the volume you listen at and the one I might listen at. I listen, day in and day out, at 75db to 80db SPL at my listening position. That gives me what TLS Guy refers to as a natural conversation level.