Omitting the obvious (ie electric impedance), there seems to be no specs for "Acoustic Impedance" for headphones. I have big ears, so it difficult to find in-ear headphones that would match voluminous ear canals. They sound better if I press them very deep into the canal, as the volume is more close to "normal human ear" and impedance matching better.
As per the Jacobis "Maximum power transfer Law" input impedance should be equal to output impedence. So impedance matching is required.