Flowmaster 40s sound EXCELLENT with an o/r - h.
My last setup was a mac o/r -h into flowmaster 40s. I couldn't leave my house without someone telling me how good my car sounded. Eventually I'm sure I will go back to this same setup.
My car had an SLP cat back and sounded awful with an O/R X. Welded in magnaflow mufflers and is sounded perfect...