Spikes or rubber feet.

btw,

I put tower speakers that could easily tip over if only 3 spikes (or my current 3 wheels). Trick is, put blocks in both rear corners, just a bit of clearance so the rear spike/wheel only is supporting the speaker.

Then, for any reason (fat parrot gets out of the cage, perches on a rear corner) if it starts to tip, the wood block prevents it from going over.

Rear spike height fixed, adjust two front spikes for level/straight appearance

Like others have said, just swap them out and determine which one sounds better.

Fat parrot made me smile for the first time in a week.

@gphill 

I called Aerial and talked to Michael Kelly about this very issue. He recommended that I use the spikes since my floor was not a slab but a floor with joists over a basement.

Interesting since it is usually the other way around.  Decoupling typically sounds better over a suspended floor (i.e., over wood joists).     As some have mentioned, Herbies makes some decoupling products.  If I still had my system set up on the upper level of our house, I would consider trying these, which came up recently in another thread.
http://avroomservice.com/evp-2/

I have owned two different types of Aerial speakers and in both cases used Sound Anchor stands to increase the weight and improve stability (i.e, the stands extend out wider than the speaker base).  You could used either spikes or decoupling materials below the SA stands.

Find out what gauge thread the inserts in your speakers use, and then buy some Herbie's threaded gliders.