Andy,
This bears out the fact, exactly as Laurie explains it, that the speakers are an extension of the output stage, and the output stage is in fact crucial to the operation of the voltage amplifier stage on a feedback amplifier. They must all work together, and if phase shifts are abrupt, or too large, first imaging and ultimately resolution will be affected.
One of the most sensitive areas is the crossover point, around 2600 Hz on the VSonics. There should be virtually no difference in phase shift between the drivers, otherwise there will be a lack of image accuracy and resolution at this frequency. You will hear the crossover frequency, IOW.
I have already mentioned the importance of making the speaker/crossover as unreactive as possible, so as not to present reactive, out of phase reflections back to the amplifier, thus upsetting the feedback loop. This too is critical and comes back largely to the Zobels which null the voice coil inductance.
Cheers,
Hugh