Arthur, those subs with L/R inputs are providing for possible use with receivers or separate preamplifiers that have two outputs, but not a single output, as most, including yours, do. Inside the sub amplifier the two inputs are combined, and there's no "stereo" factor. When the receiver and sub have a single output and input that's what should be used.
As to the sound of the M22s, particularly in the bass, "burn-in" isn't going to help because it doesn't exist, but hopefully you'll become more familiar with the Axiom sound with time. The M22s are by no means "thin" unless the comparison is with a lesser speaker that has a bloated bass response(which can be simulated to some extent by adding bass boost with the tone control). With room reinforcement my M22s have good response down to about 50Hz and much weaker, but still usable, response to about 40Hz(this is from both measurements and listening). As was said previously, this is adequate for most music, although for music having unusually low frequency content(e.g., pipe organ)and for occasional low frequency effects in movies a sub is highly desirable.
