very interesting theory ... do you know how fast sound travels though wood? do you think wood could vibrate at every frequency? versus just one?
its not crystal..
Do you have your HSU in 16hz Tune mode?
It could be your HSU may be playing louder than the XS30 on some frequencies (if I had to guess; more output between 16-22hz). IIRC, the HSU is close to the Outlaw EX, but has a bit more output. If this is the case, it could be that the content your playing is highlighting those frequencies where the HSU has more output; and causing the ripples in the water.
Your XS30 will really shine from 30hz and up compared to the HSU, and could be giving you that tactile midbass slam rollercoaster ride; where the HSU might be perceived as 'boomy' (relatively speaking) during the same content.