Gallo Ref3 - 2nd voice coil

I don't know why you couldn't use the other 2 channels, but I THINK you'd need to drive the active crossover/low-pass filter with full-range signals. You'd then send the low-frequency signals to the poweramp-inputs. The LP-filter would have the necessary level and perhaps equalization controls, too.

So, for the price of 2 pairs of interconnect cables, 1 pair of speaker cables, and 1 active low-pass filter, you have the equivalent of the Gallo bass amp/filter.
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You must use a cross over to run the second voice coil, to augment the bass where the natural roll-off occurs (around 40hz). I'm using an Outlaw Audio ICBM, but you can use any low-pass filter.