The subsonic filter is a high-pass filter. It cuts the lowest parts of the audio spectrum. I don't know what frequency Axiom picked for theirs though (15 Hz?).
Quite a few recordings have noise in the very low bass range. It isn't meant to be there, but it wasn't filtered out of the master because no one heard it on the studio monitors. Now that subs are reaching the infrasonic (proper term, as subsonic means slower than the speed of sound, while infrasonic means below the hearing threshold) range people are starting to notice this low-end noise.
EDIT: It's also useful for people who still use mechanical pickup mechanisms for playback which are liable to generate infrasonic rumble.
Edited by ClubNeon (07/31/09 04:44 PM)
Pioneer VSX-1018AH-K, PDP-5020FD, DV-79AVi
Axiom M22s, VP150, QS8s
Sony PS3, surround backs