1. I think 4 ohms is most common
2.
3. I have two, so it is well balance when on the quiet side and you can really feel them when turned up
4. I have a 4 channel amp for two subs, previous owner ran it like this since he built it and so am I
5. sealed box, I think a ported box is just louder
6. face them towards the back of the car, I have that and tried forward and the sound louder facing backwards
7. 12's seem decent
