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.