Speakers that are in cabinets loaded this way also (can) have flatter impedance curves, making them easy to drive. It is not unusual to see impedance peaks to 64 ohms at the bass resonance freq, whereas the Dynacos peak at just above 16Ω IIRC, about a quarter of the spread seen in many systems. The best known example, the Dynaco A-25, was tuned by adjusting the amount of fiberglass in a slot loaded port while feeding a low frequency square wave through the system, and was renowned for its tight, well-controlled bass. This may have been just as much due to the easy load it presented to the driving amp as acoustic factors.