Yeah, Mike; the ICBM has a standard 2nd order 12dB/octave slope which you're likely using(there's an optional 36dB/octave setting for certain subs). Typical HT receivers use a 24dB/octave slope for the sub.
So, the point Jason mentions is valid: there's nothing unusual in the sub cone movement you observed with some music at very high listening levels. You're getting significant sub output to at least 70Hz with the 40Hz crossover. Some of the instruments used in alternative rock have their lowest fundamental around 40Hz; for example, a double bass(or the electric bass guitar usually used in rock groups)has its lowest fundamental at 41.2Hz. Kick drums are another example with outputs around 40-70Hz. At high levels those frequencies certainly move sub cones, and explain your observation.
Incidentally, if you want to hear how "musical" the lowest bass notes alone played by the sub are(not),keep the settings the same(or even select the 36dB/octave slope)and disconnect the speaker wires so that the sub alone is playing during these bass-heavy tracks.