One great way to add bass to the 57 is with a pair of Magneplanar Tympani bass panels. They take a fair amount of floor space, however---each is 32"w x 6'h! That's what Harry Pearson mated with the m/t panels of the Infinity IRS to create his hybrid super-speaker.
Another way is with the GR Research OB/Dipole Subwoofer, the world's only Open Baffle/Dipole Servo-Feedback Subwoofer. Very special, and very good with planar loudspeakers. But it's available only as a DIY kit. Worth the effort!