I have to admit that I only found out that this was a double decker theatre after I visited in that there is literally a theatre (Winter Garden) stacked on top of another (Elgin). When I visited, I didn't understand why there were two names.
I saw a show (Souzatzka) at the Elgin theatre. It's historic, ornate and beautifully maintained and probably the nicest theatre I've been to. I was fortunate enough to have very good seats, so I can't comment too much on the acoustics as they weren't really tested from where I was seated. Being a proper, historic theatre however, I imagine that they're very good.
The seating was quite comfortable and they don't have to rows pushed together too tight which is important for tall people such as myself.