3 chambered flowmasters means just that...Flowmasters are a chambered muffler, which means there are baffles inside the muffler that change the tone and loudness of the sound by cancelling sound waves as they bounce around inside the muffler...and your flows have 3 chambers which means there's a lot of sound cancelling going on in there to make them quieter...My flows are 2 chamber and are noticably louder....
On the other hand, Magnaflow and Bassani are both straight through design mufflers...You can literally look down the inside of the muffler and see the other end...They use fiberglass and other deadening materials to quiet things down...Straight throughs seem to make more power than chambered mufflers, but that is a big point of debate...
As for sound, for the most part, chambered mufflers sound better with an h-pipe, where straight throughs give a better sound with x-pipes...