You should get this Borla exhaust. It is a Dual Exhaust 50 State Legal CARB Approved. So you can be louder, but legal. No need to be silly fart-can loud either.
http://www.trdsparks.com/displayparts.php?Mdl_dtl=570&Part_cat=2&parts_id=1958
Borla’s high-performance Exhaust System is now available for the 2008 Scion XB! This rear-exit system offers you aggressive motor sport styling with street commanding sound and maximum flow for more power. It has ultra smooth mandrel bends made from T-304 stainless steel and all-welded construction for durability. The angled polished tips add a high quality finishing touch to your xB.
http://www.trdsparks.com/displayparts.php?Mdl_dtl=570&Part_cat=2&parts_id=1958
Borla’s high-performance Exhaust System is now available for the 2008 Scion XB! This rear-exit system offers you aggressive motor sport styling with street commanding sound and maximum flow for more power. It has ultra smooth mandrel bends made from T-304 stainless steel and all-welded construction for durability. The angled polished tips add a high quality finishing touch to your xB.