Some good advice here from Brian, Phil and others (you know who you are). I don't want to take on the entire discussion and only have one tiny area to contribute.

Sometimes its necessary due to a budget... However, anytime you run the bow speakers off the source unit it forces you to run the gains on the amplifier (that runs the sub and four cockpit coaxials) wide open or at least inordinately high. You have to do this in order to keep the source unit internal power from prematurely clipping. And now its going to be difficult to run the tower speakers more aggressively off the same amplifier. So if you power the bow speakers from the external amplifier also you can achieve a better tuned, cleaner and more balanced system. You could either add a very small 2-channel for the bow or a larger 2-channel for the tower. Since your sub channel(s) and tower channels usually are worked the hardest I prefer to break these areas into separate amplifiers. So that's the story and I'm sticking to it.

Earmark Marine