The input of a powered sub is the input of its internal amplifier, which will have a very high impedance, 10K in the case of at least some of the ML subs. That means the sub will draw negligible current from the main amp, and therefore have no effect on the main amp.
My instinct would be to connect the sub to the same taps as you will be using for the main speakers, presumably the 8 ohm taps.
Although connecting the sub to the 4 ohm taps while connecting the main speakers to the 8 ohm taps might work equally well. The only difference that occurs to me is that using the 4 ohm taps for the sub would result in a signal level being provided to the sub that is about 3db less than what the 8 ohm tap would provide. That would be compensated for with the sub's level adjustment.