There are many things to like about high-efficiency speakers—such as their use of simple crossovers and their compatibility with low-powered tube amps—but there’s a big catch: integrating drivers without relying on complex crossovers, all while keeping capacitance low and impedance high, is a real challenge.
Speaker sensitivity ratings often cause confusion because they’re typically stated based on 2.83V input rather than the 1-watt standard. While 2.83V equals 1 watt into 8 ohms, most high-efficiency speakers are 4 ohms. In that case, 2.83V into 4 ohms equals 2 watts. To measure correctly at 1 watt for a 4-ohm speaker, the input should be 2V.
If you’re using solid-state amplification, this usually isn’t an issue, since a properly designed solid-state amp doubles its output power when the load drops from 8 ohms to 4 ohms. But with tube amps, output power remains (almost) constant regardless of load. This means that a speaker rated at 90dB sensitivity and 4 ohms is effectively 87dB from the perspective of a tube amp.
In my experience, as a general rule of thumb, small speakers typically require big, powerful amps, while large speakers can often perform well with low-powered amps.