Quiet mufflers

Most quiet and least restrictive is really not doable. I just replaced my Flowmaster 50s with Dynomax Ultraflow, primarily to reduce the backpressure but also hoping to reduce the noise. They did both. Flowmasters are made to be noisey! The 40 series are louder than the 50 series but they are all still loud. What surprised me in my research is that they are also about the most restrictive "performance" mufflers you can get. Baffled mufflers create restriction .... period. To avoid restriction you need to go to a straight through design and those that use pulse dampening material (glass packs) can also be more quiet plus have less restriction.
My idle jumped up over 200 rpm and the vacuum level created by my header evacuation sytem also increased significantly - by changing from the Flow 50s to the Dynomax Ultraflow - and they toned down the sound at the same time.