X3 on the Flowmaster slimlines... They are the quietest, low restriction sidepipe mufflers you are going to find.
Expensive, yes, but if noise reduction is your primary goal (without killing HP), then you will not find a better sidepipe muffler.
and, as an added benefit, the insulating wrap is a thermal barrier fabric, so they don't get nearly as hot, either... great for the backs of your legs.
Here's a link to another discussion where one of the California guys switched to the Slimlines on his ClassicRoadsters:
Question about rattle in CR sidepipe
Now, if you want a much cheaper muffler option, that will reduce DB, but will also kill off 50-80 HP, then go to Jones exhaust, and order a set of their turbo-tube mufflers - these mufflers have a pair of opposite flowing internal baffles that shave off about 30Db in all RPM ranges, but the internal baffles are only 1.25 inch diameter, so they flow like absolute crap... Not recommended