How do you listen?

There seems to be a consensus-75 to 80ish db.
That is exactly how I listen. Going to the symphony, I have never had the experience of feeling the sound was too loud. Unlike listening to my system.
Rock concerts are a different experience. Especially close to the PA.
Maybe that is similar to being close to my system speakers?

More or less I feel it depends on the way the album was cut and engineering...some sound louder than others. I always turn down the volume and then renew the sound...I never use headphones, I only used them back in the 60s n 70s when I lived at home...I believe they are the number cause of hearing damage.....so I don't  use them....everyone who has a earphone in your ear 18 hours a day or listen to music it loud on your headphones, you cant get your hearing back...oh you can get $8,000 ear phones....I have gone to plenty of concerts and sat up close and remember after the concert everyone was saying what ? Over and over...lol

Right now I am listening to jazz.. about 75db.

My symphony seats are 8th row center (on aisle on left). The crescendos are too loud. But when there is a soloist the violin sound hole or piano sounding board is pointed at me. This makes the solos amazing and perfectly blended with the orchestra. It also allows different instruments to be clearly discernible while allowing for complete integration when large parts of the orchestra play simultaneously. Also, my line os sight is above the stage so I can see the players.  So, very occasional over saturation these are the perfect seats in the house. 
The general rule is 7th row center, but over time I found these a tad too close in symphony hall I attend.

When measuring the volume level you typically listen to your system, is there a consensus on which frequency weighting to use? I find that C weighting can display a reading between 5-10 dB higher than A weighting depending on how much content below 500 HZ is present in any given piece of music.  

Never on headphones, due to tinnitus. 
Volume depends on the disc and the material.