How to hook JL F112 sub to both preamp and pre/pro

OK, so I impulsively bought an F112 sub. WOW. Great for movies via LFE sub out from Sunfire pre/pro. However, cant engage sub when I use source direct mode, which bypasses all processing. Sub only has line level input. Been thinking about buying separate 2 chnl reamp with HT bypass anyway, but not sure how I would hook that in. Maybe a line level switch box, where I would send the LFE out from the pre/pro and also sub out or second set of L/R output from preamp ansd switch them as needed? Is there such an animal, or any other ideas? Thanks...Jeff