Thank you, mah. This article will require considerable study, but looks to be worth the effort. I sure like the term "transient perfect" and the various square wave responses look a lot like what I have heard, i.e. clean vs. scrambled. If I understand the author correctly, the "2nd Order Transient Perfect" crossover connects the drivers in the same polarity with aligned acoustic centers and fills the response hole at crossover by overlapping the responses somewhat. Nice!