spikes under a subwoofer ?

I think what you need are the Auralex SubDude HD Isolation Riser (the HD means it's dressed up a bit for home use v. the regular SubDude). Widely used in homes and pro studios. These are foam type supports to use under subs (there are speaker and desk versions also). The things really work!

Kal Rubinson did a review in Stereophile in about 2005 or 2006, and it was basically a rave review. Absorbed vibrations, the house and bookshelves no longer vibrated, and the sound was improved considerably.

They're for sale on various sites (Audio Advisor, Music Direct), where you can also read numerous user reviews, which are something like 98% very positive, even raves. Cost is about $60 for the sub version.

In the case of the sub dude,if you want to take it to a higher level...a must have is the sub-trap made by ASC. I've been using one for years and it comes highly recommended...

http://www.asc-hifi.com/sub-trap.htm

One of the oldest threads I have seen come alive! However, I have two JL Audio F113's and my floor is carpet on wood with underfloor. The Auralex works great and greatly reduces the unwanted frequencies that were caused by the sub coupled to the floor which transferred into walls furniture etc... I highly recommend raising and decoupling a sub off this type of floor.

I cut 2 yoga foam blocks to make 4 smaller blocks and put them under the sub. It was too boomy sitting directly on the floor.
Sorry I can't be more technical than that.