After playing in the more modern Ford Auditorium in recent years, the Detroit Symphony Orchestra next season will move back to historic Orchestra Hall, which musicians call an acoustic gem. Orchestra Hall, built in 1919, was scheduled for demolition in 1970--to be replaced by a fast-food restaurant--when enraged patrons mounted a campaign to save it. The restoration is not yet complete and will continue in phases over the next several years, but about $2 million in renovations will be made to the hall before the orchestra starts its season on Sept. 21. The tune is not entirely upbeat in Detroit, however. The orchestra is hurting financially, and still trying to recover from a 12-week musicians strike late in 1987 that caused subscription losses, the organization’s main source of earned revenue.