Can someone tell me how much sound comes out of a open circumaural Headphone?

I'm not sure what you're asking. Are you wanting to know how loud open/closed circumaural headphones are, and how much external bleed there is from open circumaural headphones?

The volume produced out of the driver is really dependent on the headphones themselves as well as what you're listening to. As far as external bleed from open/closed, there's obviously a lot more on an open design than a closed design.

Personally I dislike open headphones and I much prefer a closed circumaural design because I like to use isolating headphones as much as possible. If you're listening in a quiet environment or around people who won't mind hearing your music as well, then open might be a good choice because they do give a truer sound--there's no distortion from resonating off of a plastic backing.

I don't know if that answered your question. 😕

In terms of soundstage the HD600s benefit form being open, but they are quite leaky, certainly no use for recording with them turned up loud, but not only do I like my hearing, I also feel that I could use cheapo phones for live monitoring and the hd600s for mono tweaks on individual tracks. As they are beautiful in that respect.

You might want to look into Grado