The basic issue seems to be that a speaker takes electrical energy and converts it to mechanical motion that in turn is converted to acoustical energy. Thus there are multiple conversions involved in getting from the electrical energy being input into a speaker, where Volts, Amperes and Watts matter, to the acoustical energy at a listener where Acoustic Watts, Sound Pressure Level, Loudness and so on are related.
Different speakers will convert the energy differently so a given electrical signal value at the input of a speaker may relate to a different acoustical signal value for the output of different speakers. How that acoustical signal couples to the room and from there to the listener may also differ.
I was just reminded yesterday that when the Beatles played in Atlanta in 1965 they performed at an outdoor stadium to 34,000 people using a total of around 700W to power both the mains and monitors (the generally acknowledged first use of stage monitors for large scale live sound reinforcement). Expectations for the sound quality may have been different then but thousands of screaming girls was just as loud then as now so being heard above that with, by today's standards, so little power required efficient speakers and shows just how much how the speaker converts the electrical energy input into acoustical energy can matter.
