Wayne Parham Messages: 18835 Registered: January 2001
Illuminati (33rd Degree)
If you search here for "flanking sub crossover" you'll find lots of threads where we discuss different crossovers and amps. Some are plate amps and some are separate crossovers and amps. But the truth is, you're right, that most plate amps offer only a fourth-order slope.
One of the exceptions is miniDSP. Not only do they provide stand-alone crossovers but they also offer plate amps. So that makes it really easy - just set it for "100Hz BW2" and you're done. Naturally, the subs need 10dB more gain than the mains too, which can be done either by attenuating the mains or boosting the subs.
Don't make 'em loud enough that you can hear them distinctly - They should be very subtle. You should be able to turn on or off the subs and notice very little difference except for more bass extension. The lower midrange will be smoother too, but that's subtle. So basically, the SPL should be set so the subs aren't noticeable. Each side becomes like a three-way speaker that sounds like a single seamless sound source.