Spintech was the quietest I used,but it clearly robbed me of LOTS of power, so I took it off. Flowmaster worked second best, but only for a couple of hours until all the packing fell out. Removed that one too. Running a long pipe to behind the seating area is acceptable. Thinking about trying multiple 1/8 holes like someone else described.
Also tried a race muffler from Burns Stainless with little effect.
Dave Leonard