The biggest problem I see is how everyone rates product differently. If you buy JL Audio for example and want amps, speakers and subs, the tech guys know that the sub rated at 100 actually can take twice that, the amp rated at 250 watts mono is actually pushing 400 watts, etc.
If you want Kicker subs with Alpine amps and JL Audio speakers, what specs do you go off of? All the guys twist the numbers their own way it seems.
With that said, if you do a little of the homework (go to forums, find out actual power ratings, power handling, etc.) than a mixed setup can sound as good or even better.
Juan