Spikes under the Subwoofer?

Someone told me that a subwoofer needs spikes under it, just like normal speakers to help improve bass and whatnot. I have an M&K sub, side firing acoustic suspension. It has some cheap looking plastic nubs for feet. I was curious what people on this site use.
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.
This is a response related to this thread that I wrote for www.audioreview.com. But their server was constantly down so I thought I would drop it in here because it relates tot the transformation from bad to good from using Vibrapods on two Infinity HPS-500s I own. So that as a backdrop here is the thread:

I was initially disappointed in this sub's lack of musicality (is that a word) in my system. At least it integrated poorly for music. I usually just shut it off. I did not think this sub needed any help with dampening, in part, because it was placed on a concrete basement covered with tile. But immediately after placing the Vibrapods (But if you use 2 #5 Vibrapods under front feet and 2 #4 Vibrapods under rear feet a miracle happens. No kidding. I have been an audiophile for 3 decades and it is rare for such a musical transformation to take place for so little money. You lose that fake added bass from the cabinet vibrating on top of itself (meaning the cabinet is vibrating long after bass note is produced from signal hence when the next bass signal comes in the cabinet partial cancels each waveform but creates a new one eventually leading to a monotone rumbling bass. But the vibrapods seem to damp out these cabinet vibrations, at least the important ones linked to cabinet and floor interaction, almost as quick as the woofer produces them. At the Vibrapods and take a MAJOR leap forward in Bass Quality (the extra rumbling bass will go away so apparent bass loadness will not be there until real low bass signal comes along)he sound was INCREDIBlY improved. Now I always leave the sub on at its lowest hz setting with about half gain for music. It really helps in my major league size listening room (25 ft x 15 ft x 8 ft). I was about to give these things away. Now I am happy with my purchase. The vibrapods will cut out the sloppy bass but leave you with apparent faster and much-much-much more articulate bass for music. Hence if you want resonanting bass to linger long past the signal for those non-sense sound effects don't do anything. This sub will deliver that type of bass foundation that keeps on going and going and going and....(ala Energizer Bunny).

The rest of system includes, in part, B&W 801 series 3 powered by Threshold SA4E Pure Class A amp, KEF 200C powered by Classe CA-100, and Matrix 805s. Air Matrix interconnects, Rotel RHT-10 tuner, Audiolab 8000t tuner, Meridian 608 Digital Tuner, Analysis plus speaker cables, 2 Monster Power HTS-5000 power conditioners, Stealth Power Cords, Audio Research LS3B Pre, Audio Research SDP1 Surround Processor, APS-13 17 ft antenna.

In summary, the HPS-500 is pretty much a disaster for music in the context of a high end system without Vibrapods but with them they are certainly above average but don't think they will give the sub leaders at several thousand dollars sleepness nights. So every bit a high end value if placed in the room correctly and on Vibrapods. I am only give the sub all 5 stars to partially off-set others that must think hi-end audio is about one upping the next guy and need to put equipment down. In reality I would rate them a 4.5 for value and a 3.5-4.1 overall (but I am being very critical here but I think objective when it comes to the very best subs out there)
What kind of flooring do you have? Is your system on the 2nd floor?
I believe Nanderson is on the money in the context of most reasonably priced subs. Personally, I prefer the use of spikes with subs, BUT - there can be an issue with downward firing subs in that the space under them is affected by the insertion of spikes, AND if cabinet resonances are significant then Vibrapods are very effective. I agree with Nanderson that the beneficial effect of Vibrapods is to remove cabinet resonance, but there is a downside too in that energy is not released quickly - although some may like this effect because it will add perceived bass weight. So if the sub is a bit suspect in the resonance area, use Vibrapods. If cabinet resonance is not an issue, use spikes. If the sub is downward-firing, be careful to maintain the manufacturers intended clearance from the floor.
spikes quicken the low frequencies,no typical bass overhang. Used in cases where a carpet is beneath the sub.
Each has individual characteristic. Same goes with those sorbothane that audioquest employs. However try out Foculpods which is cleaner sounding than Audioquest. The rule is "to hear is to believe". Note too that it has a relative weight which it is effective.So check the weight of the sub which is coupled to it.
There was a rickety old man in a rickety old wooden house with old wooden floors and a [chuckle] downward firing sub; Then there was a man with nice, solid concrete floors and a forward-firing sub; If you are into home theatre/audio, have a good, long think...before you decide where you are going to live.