I may still be missing something, but it sounds like you had the answer to your original questions in the manual.
VonSchweikert.com probably has the same information that is in your manual. On line specifications report frequency response is -6 dB at 18 Hz, -3 dB down at 25 Hz and the transmission line is coupled to the room at 23 Hz by a tuned vent. Of course, room effects and modes may cause the measured response to vary in your listening environment.