Last night I performed my latest mod to my smart. I installed Kicker 07DS650 2-way speakers in the doors to replace the originals. All I can say is WOW!!! I didn't realize what I was missing. The sound is so much clearer, and I can now CRANK IT UP. I still have the original radio 9, so I'm not driving them very hard, but it is a vast improvement over the stockers. I got them on Amazon.com for $39.99 w/free shipping!
I was originally going to install Polk DB651's, but after reading several reviews that talked about the Kicker's having more "kick" I decided to try them. I'm happy!
The install went really smooth, I left the original speaker grills to maintain the "stealth" look. Once you remove the first door panel, after that it's a breeze. Took me about 1 1/2 hours to do the first door(had to take it apart twice). The other door took 20 minutes.