I noticed the same improvement with my speakers when I replaced the factory spikes with longer ones. The factory spikes did not penetrate all the way to the concrete slab. The speakers were floating on the carpet and I could turn them slightly. After switching to longer spikes the speakers are firmly anchored as is the bass response.
Leatherneck1812, Are your speakers on spikes on top of the maple? What is between your speakers and the maple?
Leatherneck1812, Are your speakers on spikes on top of the maple? What is between your speakers and the maple?