Having used many RELs, I have to disagree with the above Sumiko recommendation. The only time it is appropriate to go without spikes is if the sub is sitting directly on a hard surface. Otherwise, you should use the spikes and adjust them so that the air gap between the cabinet and the top of the carpet is the same height as the legs. In other words, the legs should graze the top of the carpet. Thinner carpet = shorter spikes, thicker carpet = longer spikes. Putting the legs directly on carpet results in the cabinet being "suspended" by a crude spring and muddies up the bass. If the carpet is hard and thin enough to be virtually the same as a hard floor, then you can probably get by without spikes.
