Split-manifold inline sixes have more of a problem with resonance in the pipes than actual decibel volume of noise. The classic sound for 1950's dual-exhaust stovebolts involved steel-pack straight-through flow mufflers called "Smitty's", which enhanced the resonance and gave a "BRRRRAAAAAPPPPP" noise at big throttle openings. They were only mildly obnoxious at low RPM and part-throttle settings. It's hard to get just a little exhaust noise, like it's hard to be a little bit pregnant. As the muffler(s) get a little age on them they get louder.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!