It must be somewhat rich. I went from an inverted wrap around pitts style (twin pipe), to a stock (single pipe) muffler. It's now got a slight gurgle in low -to- mid range.

I'm flying a GP Ultimate 1.60 ARF at about 16.5# w/DA-50r engine, plus smoke system. It'll hover at 40-50% throttle.
Soon as I decide on which muffler to stick with (testing for smoke systems output), then I'll tune it better.