Ned, I thought about replying, but I just don't really have anything to offer here, and am sensitive to those people who object (rightly, often) that spewing conjecture or uninformed opinion does not really help.
Relative to Axiom tower speakers, I recall Alan saying that there was no audible difference between using the spikes and the rubber feet. The spikes are designed for carpet and the other for hard surfaces. As with other things in this hobby, the "right thing" is whatever sounds best to your ears in your room.
Relative to my M60's, I like the spikes because they ensure the speakers won't get moved (scooted). There are also minute (but likely discernable) issues relative to height with the tower speakers that would not be relevant with a sub.
It seems like your change from the spikes to the foam would uncouple (decouple?) the sub from your house somewhat. Again, if you like it better that way, that's all that matters.
