Mongoose if you want to sound like a tank can hear it coming for about 2 miles

and it will knock like mad on the rear beem unless you fit it perfectly
I run one on the dimma lol
The williams currently has a scorpian and its a nice sound, not to loud and no drone on the motorway
I like the Janspeed too, played with one on the cup car and it gives a very nice note with out drone.