Jack, interesting question. Remember that the speaker's natural dropoff measurement is taken with a constant signal level. When you set the receiver's crossover point, the signal going to the speaker below that point is reduced by X dB/octave. So if your crossover point is at or near the point where the speaker naturally drops off, the cumulative effect results in an even steeper slope.
Now to specifically answer the original poster's question. I am an M22 owner and also have an SVS PB12-ISD subwoofer. I cross my speakers over at 80Hz. I've tried running them freely, without a crossover, and setting my sub's crossover to blend in at 60Hz anbd below, but the sub does a much better job of providing the "oomph" that is often found in 60 - 80Hz range. The M22s excel in midrange and treble, so I prefer to let them concentrate on what they do best. I leave them crossed at 80Hz and have never looked back.
