Right, for typical prosound apps back then and still to a great extent today in touring sound, 60 Hz was/is the lower useful limit with 40 Hz being typically at least -24 dB, so its seeming too small vent for its rated power is more than adequate.

For HIFI/HT apps, tuning 12 ft^3 to Fs to as low as 17 Hz should be fine combined with some room/boundary gain and maybe some EQ, though it takes two of these to match up to a single A7's mid-bass horn's sensitivity.

GM