It's about being able to play at reference levels, which for movies involves the audio spectrum, including low-frequencies, being able to hit 105dB SPL.
A movie that calls for that level (and many do), requires some pretty massive subwoofers.
I used to have 2 Velodyne UL-18's, and
my IB sub kills those in ability.
Due to
flecther-munson curves, our hearing is less sensitive in the bass, and thus requires a speaker system that can deliver a good bit of of output at those frequencies. This means (due to physics) a rather large amount of air displacement.
Of course, this is proportional to the size room you are trying to energize, but for the avg. 2,500 cubit foot room, a minimum of 9 liters of air displacement is advisable.
A ML Descent will do fine for something like that.
Also, do not underestimate the amount of bass a well-recorded music disc will have. Even classical will place substantial demands on a bass driver.
They key is to have a sub that is
accurate.