Personally I would take two wires from the left speaker outputs on your amplifier and two from the right and connect them correctly to the speaker level inputs on the subwoofer. Of course this means you will have your stereo speakers, and the sub, connected to the speaker connectors on the back of your amp, but it should be easily doable. The wires don't need to be very thick at all, because all they're carrying is a signal to the amplifier in your sub.
On my active subwoofer the speaker level connector wires are the same as telephone wires, in other words, very thin.
The fact that it's an active subwoofer, should mean that the impedance of the sub, will be negligible therefore you won't overload the amp. Check to see what the sub's impedance is, but as I say, it should be such a large number as to mean it will make no difference to the load on your stereo integrated amplifier.
Active subwoofers, integrate with a stereo set up, much better connected this way than trying to use the line level connections.
I would try the crossover frequency at about 80 Herz, experiment with phase., to see what sounds best and have the speaker of the sub firing across, or roughly at right angle to the direction of your stereo speakers.