The only other times I've seen the Opera House that empty were Amorphis in 2008, Green Jelly in 2008 and Enslaved in 2007. But this may have been the emptiest.
08/23 - Alice in Chains
09/19 - Deafheaven
10/18 - King Diamond
11/01 - Suffocation
12/06 - Distant Worlds: the Music of Final Fantasy