1. You can take the signal from the rear speakers, but the speaker should not remain connected to the ASK.
2. If you only connect the cable (short circuit) instead of the subwoofer, you can get problems with the impedance. As far as I know, front speaker have 4 Ohm, and subwoofer 2 ohm = makes 6 ohm total.
Without a sub and only 4 ohm, you can easily overload the ASK (when you play louder
)
I recommend to disconnect the front speakers from ASK, use the front signal for Audiocontrol LC6i and put a 4-channel amplifier: 2 for the front, and 2 for the subwoofers.
The rear speakers can remain connected to the ASK.