The amps sub output can drive one or two subs, there is two + and - outs, but there are mono, so the same signal.
The amp has hi-pass on the fronts.
Hi and low pass on the rears.
Low pass on the sub outs.
I have actually crossed the fronts over at nearer 200hz, and the same on the low pass for the subs.
Because the subs are under the seat this works well, when I put a sub in the boot I will make sure the fronts are installed well enough to go down to 60hz up front so I can cross the sub over at the same point.
The crossovers on the front have a 6db slope and the sub has a 12db slope for those that like to get techy.
Your only down the road mate, if you want to hear it just shout and we can meet up on the A11 somewhere.