single voice coil
gain is amperage (volume)
crossover is what hz the amp will allow through to be played. 50hz is a little low for a 12, if you can, go about 80 to 120 or so.
The safe gain level can be found by clipping. Clipping is the maximum the sub will put out, it is bad for it and if you never hit it then that's good... if you do, you will hear a distorted noise, like a piece of paper in a fan (hard to describe, but you'll notice)
What kind of car were you in with the other sound system? Each car sounds different, and a 33 skyline is especially bad for sub acoustics as the sub sits in the boot, cut off from your ear, and there is a battery, some other shit, a plate of steel then a seat it has to get through before it gets to you...