I've been continuing to poke around in various archives to see what historical data remains about some of the early legit venues downtown.
The main Downtown Theatres page now has a section about half way down the left column listing some of the pre-1900 Los Angeles Theatres. If you have any tidbits to share about any of these, please get in touch.
One early venue of interest on Broadway was Blanchard's Music Hall at 233 S. Broadway. As was the case with a lot of early theatres, it was a second floor hall (like the Merced) with retail (in this case a music store) underneath. The 800 seat venue was a prominent stop for touring musical acts.
The building also housed quite an array of other halls and music studios. See the listing for Blanchard's on the Broadway Theatres page for more information. One bit of data I haven't found yet is a closing date for the venue.
The illustration here of the Blanchard is from Brent Dickerson's website "A Visit To Old Los Angeles." His site does a great job of showcasing an amazing collection of old postcards and photos.