in my case, they made a *significant* improvement! i'm not a big fan of "tweaks", but i truly found this one useful.
before, when my speakers were on the floor, i thought they sounded fine. after i bought the stands (40 bucks from circuit city) i immediately noticed a difference - mostly in the highs. i used to have my treble & bass at the one-o-clock position. i put them back to neutral.
i attribute the increase of highs simply because the tweeter is higher-up and on the same plane as my ears. the bass didn't seem as "boomy" either.
another added benefit is that my three front speakers are now more close on the same horizontal plane...that helped the the timbre-matching.
so, i absolutely recommend stands. by far, the best, most noticeable tweak i've done to my system.
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"The ship of death has a new captain." - nosferatu (1922)
Adjust speakers so tweeter is at ear level. Generally floor standing speakers do no need stands for this, but do make sure you use some spikes to isolate the cablinet from the carpet/floor.