I have an 2006 660. Other than a K&N air filter it is stock. What is the best way to open up the exhaust for better performance and sound? Is a slip on the way to go, or these caps that I have seen around? I am leaning toward a slip on, but would like opinions. I am new to the site, so sorry if this topic has been brought up 1000x in the past. If slip on is the way to go..which one? My budget is around $300. Thanks everyone.