If you can link up some articles that would be a great help. There may be a difference, there may not, but either way, can you hear it?

Subs have different power requirements than the rest of the system. It's also easier to sell people on the idea of one nice tidy box instead of having the separate amps and yet another set of wires running all over. Distortion and other types of introduced noise is also heard far less in subs than the rest of the setup. If amps were built into the speakers more often then maybe a case could made that way. There are still some good subs that are amplified externally.