My friend is a DIY speakers builder. He also fixes drivers.
He had and measured dozens different 15' driver JBL, Tannoy, Altec,... He says the maximal sensitivity of 15' driver in vented box (of any size) at 100Hz is 94-95dB. Some 15' drivers like Altec have higher sensitivity around 200-300Hz but it drops to 95dB at 100Hz. 10'-12' have even smaller sensitivity at 100Hz.