You bring up a good point here.
I've also wondered about the advantage/disadvantage of roving the accel vs. roving the impact.
Reciprocity would suggest it doesnt matter but I wonder if that is true from a practical sense in some structures and modes.
If I suspect very little energy would be seen at a point/direction when the impact is in another direction, is it a valid test?
For your particular case, my "guess" is that the base would probably only be of interest in the vertical direction. Perhaps an ODS would be the first order of business....just to see what motions may be causing you headaches.