I replaced only the speakers and the sound is much better. I bought the Pioneer speakers that so many others rave about. The rear speakers don't have the same volume as before, but the sound is so much better. Just do it.
2014 Mustang GT Premium: Ruby Red, GT500 exhaust/valance/spoiler and Roush Upper/Lower grills and splitters
1987 Camaro IROC-Z: 350, TTops, Leather, Bose and only 20k original miles