Try taking some of the sand out. It sounds like you might have them a tad over-damped. Experiment with the 75% filled as previously mentioned. Also, you definitely need spikes or some type of footer to couple the stands to the floor. If using spikes, you'll need special pads for the spikes so you don't ruin the wood floor. You might also try some large Isonode footers or the Herbie's big dots under the speakers. There are a lot of threads on this subject, so you could do a search for more suggestions. Have you asked the Skylan dealer what they recommend? As I recall, wood floors, depending on if they are laid over a concrete base or are suspended, require different set-ups of the speaker stands. here's an older thread you might find of interest:
http://db.audioasylum.com/cgi/m.mpl?forum=tweaks&n=62576&highlight=sand+in+speaker+stands
And another:
http://forum.audiogon.com/cgi-bin/fr.pl?cspkr&1156645142&openflup&2&4#2