even more important than impedance, at least to a stable amp design, would be the phase angle of the load.
B&W? Stereophile may have tested and measured these. Impedance mins at large phase angle indicate 'worse' load.
If the proposed speaker is better in this regard, at least electrically, it'll be a better match.
Example: Your amp would probably be OK with 4 ohm resistors, but NOT ok with an '8 ohm nominal' speaker with dips to 3 ohms, and 45degree phase angle at that frequency.
B&W? Stereophile may have tested and measured these. Impedance mins at large phase angle indicate 'worse' load.
If the proposed speaker is better in this regard, at least electrically, it'll be a better match.
Example: Your amp would probably be OK with 4 ohm resistors, but NOT ok with an '8 ohm nominal' speaker with dips to 3 ohms, and 45degree phase angle at that frequency.