Doerr's Opera House operated out of the second floor of the building on the northwest corner of 6th and Maine Streets.
http://www.idaillinois.org/cdm/ref/collection/qpl/id/1589

Image:  The northwest corner of 6th and Maine Street. The building was erected in 1868. Originally housed Doerr Department store and Doerr Opera House. In later years the building was occupied by Hosmer's Watches, Clocks & Jewelry and finally J. C. Penny department store. The building was destroyed in a fire in the 1990's. Text on Photograph reads: Doerr's Opera House. BW Exterior 7" x 5". \Courtesy Quincy Public Library