Full range headphones, using a single driver without a crossover for all frequencies, can tell us what a loudspeaker should sound like, at least in amplitude and temporal terms. So do some planar speakers, such as the Eminent Technology LFT-8b, which has a single driver reproducing 180Hz to 10kHz, without a crossover in that range. The x/o at 180Hz to a dynamic woofer, and at 10kHz to a ribbon tweeter, are both 1st order-6dB/octave, just as in Vandersteen designs.
Polarity of speaker drivers
Many speakers do NOT have all the drivers aligned in the same polarity. This is seen on many of the stereophile speaker measurements. In certain designs, I guess this is done for better summing of driver output. Is the time domain compromise audible?