The Kenwood KAC 626 is a 50x2 or 150x1 (4 ohm loads) Your HU is decent enought to run the mids/highs. I would use the amp for the sub alone. Meanwhile save money for another 2 channel amp to run your mids/highs in the future. Backfill is really not necessary in our cars.. if anything they only ruin the soundstage by bringing it to the back. Get a good set of components for the front in the future.