The speaker terminals act more like an extension cord, as you put it. There is no HPF using the speaker terminals on the subwoofer. The only advantage to using the speaker terminals in your instance would be if you want to cross the subwoofer higher than the 90 Hz roll off of the subwoofer output on the amp, because the speaker output of the amp is full range. I don't think there is a way to HPF the speakers with that amp. If you use a LPF/crossover setting on the subwoofer below 90Hz then I would use the pre-amp out. Any higher setting on the sub using the pre-amp out would not make any difference as the pre-amp out already cuts off everything above 100Hz.