Cat, I believe the mellowing is due to carbon buildup inside the baffles, absorbing some of the sound.
I've got about 5000 miles on mine, and love them. And I know they're not too loud, sitting next to a buddies 1200 Sporty with drag pipes, mine gets buried in the noise. That's a good thing (when the law is around!) I have everything dialed in now with jets/needles, and get 44mpg, occasional decel popping (more of a rumble), and the occasional backfire, but I'm done messing with that stuff, it runs great, gets good mileage, I'm happy. I've had a bit of problem with rust on the welds (the only real drawback to these pipes) and a loose rivet (that tightens up quickly as the pipes warm up), but have found an easy solution to that. Krylon (I think it is Krylon) makes a "chrome" paint that oxidizes rust, so I painted the welds with a little of this, and it is almost indistinguishable from the normal weld appearance.
