low frequency

With AMTs, I think the folding is also an issue at low frequencies. You only have so much excursion capability before things run into each other. Seems like I've read they can get buzzy/rattly if driven too low also, but I could be misremembering. The smaller ones tend to have resonances up where a tweeter would, so that's going to be a significant limit. The diaphragm free span tends to be small, which pushes up resonance frequency, even when tension is light.
Dipoles generate velocity waves, boxed speakers generate pressure waves. The rolloff of a dipole is 6dB/octave, that of a closed box is 12dB/octave. With dipoles, you are sitting in the path of the front-to-rear waves, whereas with boxed woofers the room modes have more influence on LF perception. So a dipole woofer is able to generate low frequencies, just differently. Perhaps is doesn't have the same punch as a boxed woofer.
There are figure-of-eight (dipole) condenser microphones capable of picking up low frequencies. There is no theoretical reason for dipole loudspeakers not being able to do so.