A bass-reflex speaker is unloaded below the Helmholtz frequency. A transmission line is unloaded below its quarter-wave frequency. And a horn is unloaded below its flare frequency. This is particularly true for back-loaded horns, which act very much like bass-reflex cabinets or transmission lines in this regard. In each of these cases, excursion goes up rapidly when frequency drops below their passbands.As for your comments about maximum SPL, here's a chart that will help quantify matters. Below is an SPL chart based on Fostex specs. Add 6dB for π/2 radiation, where applicable. I've included maximum output based on the 10log figure, which of course assumes that X-max is not an issue and that there is no compression. In other words, the maximum SPL listed here is rather optimistic.
[Model][size][SPL at 1W/1M][Max power][Max SPL at 1M][Max SPL at 10 feet][Max SPL at 15 feet]
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F83E 3" 88dB 10watts 98.00dB 88.25dB 84.75dB
F120A 5" 89dB 30watts 103.75dB 94.00dB 90.50dB
F200A 8" 90dB 80watts 109.00dB 99.25dB 95.75dB
FE87E 3" 89dB 15watts 100.75dB 91.00dB 87.50dB
FE103E 4" 89dB 15watts 100.75dB 91.00dB 87.50dB
FE107E 4" 90dB 15watts 101.75dB 92.00dB 88.50dB
FE108EΣ 4" 90dB 24watts 103.75dB 94.00dB 90.50dB
FE126E 5" 93dB 45watts 109.50dB 99.75dB 96.25dB
FE127E 4.7" 91dB 45watts 107.50dB 97.75dB 94.25dB
FE166E 6" 94dB 65watts 112.15dB 102.40dB 98.90dB
FE167E 6" 94dB 65watts 112.15dB 102.40dB 98.90dB
FE168EΣ 6.5" 94.5dB 80watts 113.50dB 103.75dB 100.25dB
FE206E 8" 96dB 90watts 115.50dB 105.75dB 102.25dB
FE207E 8" 95dB 90watts 114.50dB 104.75dB 101.25dB
FE208EΣ 8" 97dB 120watts 117.75dB 108.00dB 104.50dB
FF125K 4.5" 92dB 50watts 109.00dB 99.25dB 95.75dB
FF165K 6.5" 94dB 70watts 112.50dB 102.75dB 99.25dB
FF225K 8" 96dB 100watts 116.00dB 106.25dB 102.75dB