I just installed a pair of Kicker 07DS650's in my smart, if your looking for a speaker to give you some "kick" without spending a lot of money, I'd recommend these. Got mine off Amazon.com $39.99 + free shipping.
I know, you serious audiophiles will probably flame me over these, but I am completely happy with them and my stock radio 9. They remain clear up to the point my ears hurt....that's good enough for me. Turn it up to about 15, and I can feel the sound waves shaking my pant legs. What more do you want?
By the way, I mounted mine behind the stock grills, for the stealth look as well.