Spikes under the Subwoofer?

It really depends on the subwoofer. You may want to check with the manufacturer or a knowledgeable dealer. For example the REL Q subs like the Q50 and Q100E do not have feet. There are holes on the bottom for spikes, however the sub was designed to be played sitting on the floor. The holes for spikes were only intended for the rare location with an overly flexable floor (like an older Victorian home) that is easily set off by the sub, making too much BOOM.

I use Vibrapods between my subwoofer and wood floor. They do a nice job of firming up the bass. I have an oriental rug covering most of the floor, so I put the subwoofer along the edge where there's no rug.

i use vibrapods also w/ my hsu vtf2 sub ..dome side up

they work like a charm

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.