ive had 3 x-pipes on my car BBK, Bassani(current), and flowtech.
the flowtech had the smoothest sound by far, if you look at the design of the flowtech one, its X section is right under the trans, and i dont know what it was but those pipes sounded so good... the only downside was the cheap flanges, but i think it was only like 120 bucks back in the day