[kaigoss69]

I think your approach is backwards. You are trying to match the interior speaker outputs to the sub whereas you should match the sub(s) to the interior speakers. In the end, I would be very surprised if you required anything more than one 4 channel amp, the MS-8, an additional L7 tweeter for the center channel, and one 12" sub in the trunk to achieve competition level SQ, and enough volume to make your ears bleed, if you really wanted to.