why spikes under speakers???

When a speaker produces sound, a driver moves back and forth. It takes energy to move that driver. If an enclosure is not secured to the ground in some way, the moving driver will also cause the cabinet to move slightly. When the driver causes the cabinet to move, it is using some of it's energy to move the cabinet instead of moving air. You want the cabinet to be secured to the floor as best as possible. Try pushing over a speaker that is sitting flat on the floor and then try it with a speaker properly set up with spikes. If it's done correctly, there should be no movement. Without spikes, the bass will be more limited than with spikes. It does more than that though. It has nothing to do with air underneath the speakers. Otherwise, stand mount speakers would be worth nothing.

The drivers in a speaker cabinet will be positioned more ridgedly with spikes underneath, probably making them able to eek out slightly more detail.

Will

Spikes are, by definition, pointy, so the contact pressure between the spike and the floor is much higher than if the speaker were supported by a large area, like the wooden bottom of the box. Therefore, the spikes will dig through any soft floor, such as carpet, until they hit something hard and unyielding, like wood or concrete. A speaker on spikes will have a solidly-mounted feel to it. Audiophiles who believe that woofers can move enclosures think this is important. (Actually, woofers don't move enclosures, as can easily be determined by hanging a speaker with cables). If there is any advantage to spikes I would bet that the resulting air space under the speaker acoustically decouples the speaker from the floor surface.