I had Spintechs on a 2 1/2" system on a mild 440. The drone was unbearable between 1700rpm and 2300rpm. It really sucked for driving in town as that's the rpm range for 35 to 45 mph.
I suspect the drone vibrations came from the sides of the mufflers as the top and bottom are joined together by the welded forms inside the mufflers. I'm thinking of welding some angle iron to the sides of the mufflers to stiffen them up. Think it will work?
Also, decreasing the tailpipe diameter will lower the noise a little.