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