Think of it as surface area. As the distance doubles the surface area quadruples...equal to -6dB/unit surface area.
I didn't read anyone's reminder that sensitivity values are usually expressed at 1k, which can be far less meaningful unless one knows the low frequency rolloff (and impedence). Since it's possible for a "high" efficiency speaker to have much lower response, let's say, at 50Hz than a lower efficiency one with flatter bass, one can indeed be fooled into predicting that a low power amp might have an easier time with the former...especially if a low impedence couples with a large phase angle diff in the bass. So it's not so simple...especially with a tube amp.
I didn't read anyone's reminder that sensitivity values are usually expressed at 1k, which can be far less meaningful unless one knows the low frequency rolloff (and impedence). Since it's possible for a "high" efficiency speaker to have much lower response, let's say, at 50Hz than a lower efficiency one with flatter bass, one can indeed be fooled into predicting that a low power amp might have an easier time with the former...especially if a low impedence couples with a large phase angle diff in the bass. So it's not so simple...especially with a tube amp.