The Los Angeles Philharmonic won’t officially open its new Frank Gehry-designed digs until the fall, but a special open rehearsal/performance by the Phil this week allowed musicians and critics their first shot at assessing the acoustics of the much-anticipated space. The hall will have to be “tuned,” of course, a process which will last months if not years, but Mark Swed was encouraged by the “plentiful bass, crystalline clarity and forceful immediacy” of the orchestra’s new home.