Speaker tweak

Double Ziplock bags filled with fine grained sand works well. These can be put into cloth bags if aesthetics are an issue.

You are hearing the effects of damping / altering cabinet resonances. The problem with using something solid to damp resonance is that it too has a specific resonance. As such, you've traded one resonance for another. Try using sandbags atop the speaker as Frank suggests. Unlike the solid mass that most people use, the sand shifts within the bag as it is vibrated. This dissipates energy as heat rather than adding yet another resonance.

In order for this to be effective and not messy, i would suggest filling the bags to the point of having solid weight but not packing them full. If the sand can not shift in the bag, you'll end up with something pretty close to the solid object that you were trying to avoid using. I would use locking freezer bags for this, as they are both good sized and slightly thicker than regular bags. Fill one bag up and then insert it into the second bag. The top ( zip side ) should be at the bottom of the other bag i.e. reversed in direction. If the inner bag leaks due to the zip locked seal breaking, it has to work its' way through the other bag and seal at the other end before leaking out.

This also allows you to experiment with various amounts of weight in a very easy to add or subtract form. I would find the amount of mass that you want to add with the first bag before putting the second outer bag on. Otherwise you have to take the secondary bag off and on each time you change the amount of sand. Sean
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Thanks for the advice folks. Theo, I'm using Mapleshades Heavyhats on my amp, preamp and cd player and it does help. I'll give the sand bags a try. You know, these tweaks are what makes people think we're all nuts. But damn, once you hear those subtle improvements you can't stop yourself. Thanks again.

I heard a demonstration at CES with Harmonic Resolution Systems plates on top of Totem Speakers. These plates made much more positive effects than by just mass loading. The 4.5" X 9.5" plates that were demonstrated were around $125 each, per plate. You may wish to look into a pair? I'm ordering a pair within a week.

Couple your speakers to the floor as best as possible. Spikes should penetrate to something solid under any carpet.
As mention above, sand is the way to go. It also puts more pressure on the spikes.