There is no way to disable the 80Hz high-pass on the left and right channels of the SMS-1. You could probably set up one of the SMS-1's presets to disable the subwoofer (set its volume to zero, for example), but you'll lose that sub-80Hz information for your mains.

Since the SMS-1 leaves the left and right channels in the analog domain, I wouldn't expect it to be a problem for audio quality, but I've never tried it (my SMS-1 just gets a subwoofer signal).