Re: breathing and eyeglass mount...yes, I've found that I have to refrain from the big "sighs" that I do when I hear some idiot drunkenly talking near me! It helps if you surround yourself with actual music fans who want to listen to music and not each others' gossip. Gotta love the dimwits who - when given the universal sign for be quiet (finger over pursed lips) - decide to come closer and ask you what you're saying...right into the mics...
The quietest concert environments I've ever witnessed were
consistently orchestral so I wouldn't worry about it.
So not saying that I do or don't, but if I were going to record the symphony; based on the rock shows and symphonic shows I attend (both in the 2k attendee range and sitting in favorable seats at both), I'd run an additional 15-20db of gain over what I'd run at a normal OTS/Section rock show (
maybe 25db more gain than what I'd run at a really loud club). I find that I'd consider wearing 9db earplugs at the symphony on occasion, but I've easily gone without while my 25db plugs are a staple at all rock shows. Applause between movements will most likely clip if you aren't using a limiter but that's not the issue.
Oh, and I wouldn't check my gear or levels while recording but I think that's a given... "Checking your cellphone" is frowned upon.