Hi Jim,
As I understand this series, the DTH4215 cab is actually a 2.5 way system with a horn (separately biampable), and a 15 linked to another "sub" 15 which has a (hard-wired, non-bypassable) first order LP inductor @ 200 Hz.
So, if Brad is using a 3-way crossover prior to the amps and sending the mids and highs to the 4215's, he has eliminated that second "sub only" 15 by HP-ing before the LP. The article says add subs to this system by sending full range to the 4215 and only LP-ing your subs. For biamp operation *only the horn* is separated, so a midband to the 15's still eliminates that second "sub" woof.
Brad,
Is this what you are doing? Does any of this make sense?
Dave