I tried several setups on my Bonnie. First removed the silencers and only used the exhaust pipes. The sound was impressive, but the flames even more

Then I used a saw on the original silencers and cut them just before the first baffle so I ended up with two totally open megaphones. I think this produced the loudest sound I could get from this bike. It didn't make my neighbours very happy though...
The best sounding bike I have now is the ྄ Guzzi V7Sport, it's not the loudness, but the sound itself. I also love the sound it makes when I close the throttle.