Thanks, I think you are saying don't buy them they won't work. I have a pair of 18" Eminence subs that we will install as a starting place. Thanks BM
The EV FH may go as low as your Eminence 18" subs, depending on the box tuning.
Although 55 Hz is not very low by today's standards, there were plenty of battle scenes in movies covered by cabinets that did not go that low.
Also, using a speaker like the Lab 15 rather than an EV 15 B will allow the native response to go down in to the 25Hz range, although at a lower level than the approximately 55 Hz horn reinforcement.
Although Tim is correct that the driver won't change the horn Fc, below Fc, the driver will respond as it would in a small sealed cabinet, so using a driver that goes low (Lab 15 has a 27.82 Hz Fs and 11.8 mm Xmax) and has large Xmax (an EV 15B has a 43 Hz Fs and only 3.3mm Xmax) can make a huge difference.
Around 10 dB more output at 55 Hz (more than twice as loud), and probably 18 dB more at 30 Hz.