I went to the San Francisco Symphony last week on Friday night to hear conductor Michael Tilson Thomas. The seats were roughly 80% filled. This performance is usually sold out. I was able to buy tickets at a discount.
I don't really know Symphony finance and I don't know what experiments are being done in the symphony world to fill-up concert seats.
I could see a simple solution to the San Francisco Symphony partially filled hall
Two things seem to be the needed. First, as the time of the concert approaches, ticket sales should show rapid declines in price to fill up the remaining seats. This is known as marginal pricing and has been used by airlines to fill themselves up ( much as this makes airline travel uncomfortable for all of us in crowded coach seats).
Second, many box seats are always left empty. I think it would be appropriate, with suitable notice to the seat holders, that after the first curtain, which is usually the first complete musical piece, the seats will be given to the standing room audience.
The second point will allow symphony halls to be sold at over 110% of the seats.
You'll notice I used 110% which is often used in situations where only 100% exists. But 110% is accurate because it is 10% more than the number of seats which is 100%.