Will's Starter and Alternator at 4695 49th st n is a good place to find out about your alternator. He's built a few for me and always done a good job.
I'm guessing all of your subs are single 4 ohm coils? What i would really suggest is to sell the 2 amps, and get a single amp that works at 1ohm mono. Those amps are so different, and the JBL is such higher quality, i don't think you're going to be happy with the sound you're going to get.