there is no standard for this
always assume worst case if you want to make sure you supply enough amp power, the amp being spec’d by the speaker maker should and will obviously be delivering its power into the actual speaker load, not some nominal 8 or 4 ohm resistor/test load
so in your case, i would take the recommendation to be 40 watts into 8 ohms (and best if it could do 80w into 4)