I have to admit I was a little apprehensive, but I got a Dyna muffler on eBay for under $30 and it bolted right up on my 2012 S40.  I used a tuna can with the top and bottoms cut off as a crush sleeve/bushing and it worked like a charm. I kept the stock airbox, so I didn't bother to re-jet the carb.  It sounds sooo much better. For $30 and 15 minutes of work, I definitely recommend.

Yes I did.  It looks so much better. I tried to post a picture, but apparently it was too big.