The stock exhaust on my Norge got noticeably louder after a few hundred miles. I am not sure why. Perhaps bits in the interior burned off over time!
The Agostini silencer that I put on the Norge sounded softer with the DB killer in (but still better than stock) and noticeably louder (and even better!) than stock with the DB killer removed. The loudness never really varied for me over thousands of miles.
On my Triumph Thruxton, I replace the stock sewing-machine-sounding silencers with loud, un-baffled, raucous British Customs Predators and had the ECU re-mapped to run richer. After a few hundred miles, the Predators took on a softer, better sounding tone. Perhaps carbon built up from the engine running rich!