Just purchased the smaller JBL control SB-5 TCB once owned one around 1990 to 1994 and thou I didn't have the frequency tools back then apart from the AVP basic pink noise and film soundtracks or the odd music, mostly film soundtracks to demo the tiny sub.
I ran a few frequency sweep tests after checking firstly with pink noise and seeing it wasn't going down particularly low enough. I then thought of the stuffing one or two of the ports up. Stuffed two of the smaller ports up and ran the frequency sweep again and wow. Its now making it below 40Hz if not bit lower than that but I guess the excursion maybe a little higher so I won't overdrive it. Its got amble amplifier power. It won't do the high SPL db as the professional counterpart JBL 4782 TCB but the stuff two ports up technique sure improves. I wouldn't mind three or more of these.
Its handling well with Star Trek the motion picture (1979) laserdisc theatrical Dolby Stereo mix on DVD-RW with repeated playing of certain scenes. Some parts Jerry Goldsmith score manages down to 35Hz with shuttle pod docking with Enterprise and other scenes where Kirk walks out of the transporter room into the corridor after it malfunctioned and there's a few lower like timpani notes at 33Hz according to my ears and feeling it with spectrum lab.
Not bad for £95.00 and in good condition with tiny minor bits that I'll fix but the x4 6.5" in good working order.
Adding update as the microphones have been fitted above in the room above against some of the overhead surrounds and so I can take new frequency room readings.
Repeated the sweep with x2 JBL 4782 TCB. If I can only find way to stuff up some its ports as it has a metal grill covering each port. Or maybe attach something to cover over the ports and run the frequency sweep again.