So I returned the Document trousers in L because they were a little too big on me. They are still there and now on sale. They are the most awesome casual trouser ever and someone should buy them right now.
I grew up a symphony kid and have been to hundreds of concerts, opera performances, etc so I feel qualified to chime in. Leave the costume dress to old guys and true 'patrons'. You'll be fine in a solid suit or sportcoat with wool trousers and an overcoat. This is one of those times were if your outfit is worthy of being noticed it's not appropriate.