It just amazes me that members who don't know the first thing about crsoover design want to get into biamping.
For the upteenth time, an active crossover has to have the same slopes and crossover point as the passive crossovers, and any notch filters. If all the slopes on your passive crossover are fourth order constant voltage slopes, and no notch filters or other Eq, then that dbx unit will do the job. If not it won't.