My Flowmaster 40 cat-back has "broken in" now after 1 yr and 5000 miles and the sound is really stupendous, BECAUSE-
Just like my muscle cars-for a muffler to show its full potential and not just for performance -the secondary exhaust pipe (after muffler) has to be at least 6'-that allows the exhaust pulses to combine in a TONE !!
In drag engines you make the short exhaust pipes (depending on v6/v8 rpm), to "scavengage" the exhaust for more HP and higher rpm and with mufflers you make the exhaust pipe length for exhaust pulses for "TONE"
Bet you don't really care !! heh heh-I do, thats why mine sounds so pleasant, with rap/burgle and quiet at 60/70 mph-no drone, unless you're in the wrong gear !