I highly recommend modification to the stock muffler; it is inexpensive and sounds very good. I have one on my Box S, along with cat bypass pipes and the sound is superb. The factory muffler does not resonate or drone like an aftermarket exhaust. Any competent welder can achieve this mod for you and the cost is minimal.
http://www.pedrosgarage.com/Site_3/Make_the_Pedro_Sport_Exhaust.html