Wouldn't doing that halve the current to each output pair (or something along those lines)?
There isn't really a one size fits all answer to this question.
If we assume that you are taking 4V out of the stock Quickie and driving an amplifier with a 100K input impedance, then adding a second amplifier that also has a 100K input impedance, you will lose .17dB of signal, which is negligible.
In the case of an active subwoofer, the input impedance could be all over the road, and your amplifier's input impedance could be a lot higher. In essence, don't sweat it!