ok kid.
did you install the jumpers on the subwoofer terminals? because im going to guess that you didnt, and that you only hooked the amp up to one + and one -, and the other two are just sitting there. your amp is only seeing a 4ohm load.
so awnser the questions;
1. you only have one alpine type R, dual 4 ohm speaker - correct? model "Type-R SWR-1242D" - one 12" dual 4 ohm subwoofer.
2. what did you do with the tiny little jumper plugs that came with the speaker. are you using them?
3. did you plug in one SET of speaker wires (thats two individual wires, one + and one -) to the amp and the speaker, and then install it?
maybe we can help if you awnser those in a logical fashion.