What is your idea of "small"?
There are only a few of ways to recreate real low frequencies - high cabinet volume, high cone excursion, and gobbs of power.
Accurate "Low 20's" is going to to take a lot of all three. I would be very, very, leery of any small subwoofer claiming to reach into the 20's accurately. Hell, it takes a lot EQ'ing and power for large subs to do so without massive distortion.
There are only a few of ways to recreate real low frequencies - high cabinet volume, high cone excursion, and gobbs of power.
Accurate "Low 20's" is going to to take a lot of all three. I would be very, very, leery of any small subwoofer claiming to reach into the 20's accurately. Hell, it takes a lot EQ'ing and power for large subs to do so without massive distortion.