Good way to get the thread going again by stirring up a bit of controversy.
In my opinion the ringing of a crossover should be treated on equal footing with any driver resonances to keep it in context - look at the CSD of the total summed response and see how the crossover resonance compares in severity to the cone breakup resonances... if you can greatly reduce audible cone breakup resonances at the expense of slightly more ringing at the crossover frequency (but still considerably less than the driver resonances that remain) then I'd say that's a good tradeoff. You also have the possibility of using linear phase filters to improve matters while still achieving higher order slopes.
Of course you don't want to make the filters any steeper than necessary, because you are then making the ringing at the crossover frequency even worse for little gain in reducing out of band response of the drivers.
For this reason I don't think there's any reason to go beyond 4th order for crossovers except in exceptional circumstances, and even then I would question the choice of drivers - in most cases I think 5th order and higher is beyond the point of diminishing returns.
If you stick to wanting in-phase summing, which IMO is desirable, that basically leaves you with 2nd and 4th order acoustic crossover responses being optimal for most situations.
I couldn't agree more - but i have a question. Have you ever managed to make the crossover ringing visible at waterfall plot of a finished loudspeaker or have seen that someone managed to record it ? Thus far i have not, nor have i managed to hear it.