“My band has been using a pair of dual 15″ speakers as low end. Recently another member joined and he has a pair of single 18″ cabinets with a good power amp. We play a lot of larger dance clubs so there’s almost no way to have too much low end. We’d like to use both pairs of cabinets in larger venues, but aren’t sure how to hook them up. Our system is currently tri-amped using a 3-way crossover. Should we get a 4-way crossover and use the 18’s for sub bass, or run them in the same range as the 15’s?”
To some extent it depends on exactly what the two cabinets are designed to do. If they are both optimized for very low frequency applications (as the 18’s almost surely are) then you are probably best served to run them both all the way down. It’s possible that your dual 15 cabinets aren’t very effective down low so it could be interesting to use the 18’s to cover the range of frequencies between, say 20 Hz and 80 Hz, kind of like a hi-fi subwoofer will do. More often than not, however, I’ve found little benefit to this type of arrangement in live applications. When that bass drum hits you need a LOT of energy in the 50 Hz to 80 Hz range for it to have any impact in a large room. Try running them in the same range first. See if you can borrow a 4-way crossover and experiment sometime.
Another interesting technique is to use the 18’s as a sort of low frequency effects channel. Use an aux send or a matrix output of your mixer to get an isolated signal path. Send that signal directly to the power amp for those 18’s. Now you have a pair of low frequency cabinets that can have their own source. You can route the kick drum and bass guitar to them all the time. It’s amazing how much cleaner and tighter your low end can be with this type of setup. Or, you can bring in ‘extra’ low end from time to time. I’ve even seen live sound engineers route this send through a volume pedal to make it easy to bring it in and out. This is great for those dramatic synthesizer moments.