Back when my 6pc band did larger gigs (esp. outdoors), we used this technique to good effect. We just used AUX send controls on the mixing board, to control which channels had some signal sent to the subwoofer. Usually it was just bass guitar, kick drum, and maybe keyboards since some synths/effects do have some low-frequency 'energy' in them. We'd leave out vocals, acoustic and electric guitars, so that those signals didn't muddy up the mix (even though those typically don't have a lot of LF energy. Was fairly simple to set up, and worked OK. The AUX out for the AUX channel we used, was sent directly to the subwoofer amps. The main mix (L and R) went to those amps. Nothing much fancier than that ...
Jim