Over the years, I've crossed large-format midwoofers a variety of crossover points from 100Hz to 3kHz. Usually in a three-way system, I'm using the LF driver as a woofer and crossing between 200Hz and 500Hz. That tends to keep the woofer in its pistonic range, or within an octave of it. I tend to expect 250Hz is pistonic range and 500Hz is the first breakup mode.

In a two-way system, I'm usually crossing over much higher, in the octave where midwoofer DI and compression horn DI match - between 800Hz and 1.6kHz. In this regard, my designs are much like JBL's two-way monitor speakers that are almost always crossed over above 1kHz. The JBL 68881 is a 1.5kHz passive two-way crossover and it is a good example, being used with a 15" midwoofer and a 1" exit compression horn tweeter on a radial horn flare.

But whether a person uses two-way or three-way configurations, the drivers are going to be used in their breakup modes over a large part of the audio band. I think its important to consider all things, and drivers that are designed to be used at high frequencies usually have well-behaved cone flex patterns and don't become excessively anomalous. Lots of high-efficiency drivers are made this way, especially compression drivers and high-efficiency large format cone midrange and midwoofer drivers. I find that a well-designed midwoofer operates pretty well in the vocal range, provided its breakup modes are well damped, which is something that is easy to determine from its response graph.