I am debating and debating over amps. My latest thoughts is thus:
Use the amp in the Apline 9815 to power the rear full range
speakers. Then have a JL Audio 500/5 power a Boston Acoustics Pro
(BAP) 12.5LF sub with the 250 W RMS, the BAP 6.5" midbasses in the
doors with the 100 W RMS x 2, and the 4" and tweeter on the dash
with the 25 W RMS x 2.

I am going to play with it, but I am currently thinking that I will
want to run the 4" down to around 200 Hz. My real concern is the
25W RMS on the 4". Is 25 W RMS enough for them? I am sure that I
could turn the gain down on the 100 W RMS channels, but what is the
point in the 100 W RMS if I am only going to be using 35 of it?

Obviously I am clueless. Can someone chime in here and give me some
advice

Sam

25 is probably enough. If you raise the xover point some more,
it'll certainly be enough.

So, where would you set the xover? Are you talking the 400Hz rnage
or like 800Hz?

I'd try both and everywhere in between. It's a crucial decision to make in
terms of getting the fronts to sound like YOU want them to. I personally
like a lower crossover point (which sort of contradicts the fact that I use
dome mids, but oh well). You won't know until you try. In either case,
power requirements go down considerably as you raise the xover point.