06 Mazda 6 stereo improvement question

I have a non-bose stero and would like to improved the sound quality. Replacement speakers were recommended. Replacing the head unit is not an option, since aftermarket units don't fit in the dash, unless I buy a rather expensive replacement bezels assemble (>$300).

get a metra dash kit from Revolution Performance Motorsport for 180$

http://www.therpmstore.com/index.php?cPath=26_42&osCsid=94a424543b7f7630065d6829f8ed8180