you seem a little confused. Your sub is rated to handle 900 watts RMS total. You only have 1 alpine x 12 right? You're going to want to wire it parallel so it'll be at a 2 ohm load. You should get on you tube and check out amp dynos. Search Big d wiz and budget gem or budget bust and watch their videos. Most of their videos speak on the efficiency of the amplifier they are testing. If you already have a rf prime 1200 I'd just use that. It's fairly efficient. Definitely could do a lot worse then what you already have as far as efficiency. You got a birth sheet with that prime right. Whats it do at 2ohms? Probably over 800 watts i would imagine.