Is Spiking Necesarry?

Years ago, I brought home 3" thick granite surface plates. I placed my speakers directly on them without spikes.  Floor was concrete slab. Sounded great so I would not say that spikes are always needed.

movement of the entire box back & forth will modulate the cone movement - causing phase error and other horrible things

so you want a way to prevent that - spike or very heavy cabinets will work

or... buy electrostatics or Maggies - problem solved (and a lot of other problems are solved too)

Are the speakers on carpet or flooring? One option is to decouple the speaker from the floor by placing the spikes into isolation bases.
See Herbie's...
http://herbiesaudiolab.net/base.htm

or Decoupling Glider...
http://herbiesaudiolab.net/spkrfeet.htm

A big +1 for lowrider57!!

Mr. Clone OP, to circumvent the whole problem of continually moving the speakers around to try and get the best sound why not just cut to the chase? Use the speaker set-up track on the XLO Test CD to find the absolute best locations for both speakers. You’ll never have to move them again. Using this method on the XLO CD is much, much better than trial and error, which is really like trying to solve 4 simultaneous equations in 5 unknowns.