The only already mounted center speakers in theaters that I've worked were set up for lectures ... a
cardioid mic mounted on a
podium, usually well behind the speakers, and usually never moving from it's location. Pretty easy to avoid
feedback. This obviously does not work for body mounted lavs on actors moving about on
stage.
Fortunately in all cases the center
speaker was driven by its own amp and was
fed by a separate output
jack from the control room ... so I simply hooked it up to a
pre-fade aux bus, selected which channels I wanted to feed it, and set it to a
level that worked. With my digital board I do the exact same thing when I set up a full rig myself ... and the digital board has EQ for each output
channel so the center can be rung out separately if needed.
Regarding
LCR panning and stereo -- musicals to me are
mono, unless you have stereo music tracks that you want to
play. If I am lucky enough to have a 5
speaker surround setup then for actors I will feed all three front speakers equally (i.e. a wall of
mono) and feed the rears through a stereo aux bus equally but at -20-30db as fills. I do not pan actors.
Orchestra is pretty much the same but if the instruments are spread across the front of the
stage I may pan (still in
mono) to the left or right on the LR feeds about halfway based on where the instruments are located. Only stereo playback tracks will I
play in stereo on the LR and rear fills, and again as
mono to center. Location specific sound effects will feed whatever
speaker(s) are closest to where the sound should be coming from, if I don't have a separate sounds effects
speaker to place on
stage where needed.
And of course this is all subject to adjustment based what you are hearing in the specific
venue.
-- John