Specialty Spares recommends (if they are very settled in or you want to tone down a bit) adding a layer of 4-stroke baffle material around the original baffling. You can get it at most motorcycle shops.
I've tried no baffles - way too loud for me. Metal baffle only - good but a little raspy on the top end. I like them worn and settled in with one layer of baffle the best. If you remove all baffle material it will sound harsher (high end). The material takes that out without having a lot of effect on the low end.
