Apparent loudness changes in very subtle ways with better sound quality. With better equipment you can hear more details at the same volume settings, but it doesn't sound as loud because the ear takes many volume cues from distortion.

I had a relative over, so of course I wanted to show off my system, She said "I don't like it loud, it hurts my ears". I played at my normal listening level and she said that was fine. If fact it was probably quite a bit louder than they play their mass market system at home. Since the "uglies" were gone it didn't seem loud to her.

Radio Shack sells a cheap level meter, it's hardly the last word in scientific (or accurate) equipment, but it provides a handy reference not confused by subjective opinion.