I use spikes with my sub. It's a Genesis Servo 12 side firing. I also added weight to the interior of the cabinet. The more stable you can make the subwoofer--the better performance I've found. There are exceptions, as have been pointed out, some subs are designed to excite the floor and they become part of the "speaker"--most of these are downward firing--and in this case the loads are going up and down and spikes probably would not make much of a difference (although I have not owned a downward firing unit--so I have not tested this myself). Downward firing speakers would probably benefit from adding weight to or onto the cabinet. Side firing should benefit from both--and in my experience does.
