It is possible that the additional height that the speaker is raised off the floor by the spikes may have reduced the bass reinforcement that is normally provided by the floor. This could have reduced the bass frequencies by 3db due to the radiation into 1/2 space, instead of 3/4 space. Placing a small plate over the open space between the bottom of the speaker and the floor may restore that boundary reinforcement, and thus the bass response. It is worth a try.
The spikes generally cause the speaker to be more steady, and improves sound by reducing doppler that is caused by minor speaker rocking on an unsteady base. They can also provide a vibration evacuation path. But if the floor is very unsteady, then this may be a problem.
I agree that some experimentation is in order to find the best result in you situation. I think it is easy to place a temporary plate over the gap between floor and speaker that is created by the spikes raising the speakers off the floor. It just might do the trick for you. Boundary reinforcement is a real effect, and it can be compromised by even a couple of inches gap.
The spikes generally cause the speaker to be more steady, and improves sound by reducing doppler that is caused by minor speaker rocking on an unsteady base. They can also provide a vibration evacuation path. But if the floor is very unsteady, then this may be a problem.
I agree that some experimentation is in order to find the best result in you situation. I think it is easy to place a temporary plate over the gap between floor and speaker that is created by the spikes raising the speakers off the floor. It just might do the trick for you. Boundary reinforcement is a real effect, and it can be compromised by even a couple of inches gap.