More efficient how? In terms of converting electricity from the wall socket to the speaker terminals, yes, greatly. In terms of decibels-per-pound, yes. In terms of decibel-per-watt, no.
In the "olden days," we:
a) seldom saw tube amps that made more than 100 watts and transistor amps that made more than 200 (and they were rated by different standards, but that's another discussion);
b) didn't put all the instruments through PAs, so amps had to produce more volume; speakers and their enclosures were larger because more speaker-cone area means more decibels;
c) were more satisfied with low fidelity as long it was loud enough. The mighty SVT is not high-fi but it sure is loud;
d) didn't have drivers which could handle more than 100 watts, and as driver size decreased, their power handling decreased.
Nowadays, we like small, light, and higher fidelity, and we often ask our amps to handle lower notes than before, which require more power to be accurate and audible. Higher fidelity requires headroom, the power capacity to handle output spikes accurately. An amp rated at 100 watts is perfectly adequate until you suddenly ask for the 101st watt; how will it respond? Modern drivers can handle a lot more power than older ones, but smaller speaker area translates directly to lower volume.
