A Bass bin can make a big difference to the bottom end but with 15's in your main cabs you might do as well with a good amp and a graphic equaliser, unless you really want BIG bass.

You can get bassbins with passive crossovers (so you drive your cabs via the bin), but you'll still need a bigger amp.

Alternatively use an active crossover and a separate bass amp. This assumes you can "break into" the amp channels on your desk so that you can get the full range signal out of the desk and send back the "high" signal from the crossover.

p.s. I hope you DONT mean "Bridge!". A bridge circuit is a way of getting serious mono power out of a stereo amp, not a way of connecting two amps together so as to drive two sets of speakers from a single sound source. "Slave" is the term required, I guess.