I have a Magnaflow dual in, dual out muffler on my SEC currently, and it sounds ok, if a bit on the loud side.
That I had it built with 2.5" instead of the ~1.75" stock pipes might have something to do with the volume, as might the current lack of cats and resonators, though...

I'm planning on adding the cats, as I can't see any reason to not have them on any road going car, but haven't gotten them in just yet.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...