I tried the Gemballa muffler and one of the Dansk Sausage-style mufflers, both of which gave that resonance, which, like for others, drove me nuts. I finally got one of the Dansk Sport mufflers that look like our stock mufflers. It has a higher flow rate (I think I picked up a few ft lbs of torque, but no hp with it), but none of the resonance issues.
I like the sound, but I suspect not everyone would because is has a raspy tone. If you can find the Dansk website they have sound clips of the muffler which do give the character of the sound. Even though it can be raspy when I was behind the car on the last dyno pull I did, its amazing to hear it screem at 7,000 rpm.
Ed