I have to disagree with Paulwp. I believe your sound will be better if the speakers are coupled to the stands rather than isolated. I use "bluetack", but in a very small amount, just to prevent scratching the bottom of the speaker and keep it from sliding on the stand.
Regarding how to raise the stands: Are your current stands spiked to the floor? Can you build a small platform and transfer the spikes to the platform? Maybe use some paving stones to give it some height and weight, and place your stands on top of that.
Another idea would be to get some threaded studs the same size as your spikes, and some short lengths of tubing with the same threads on the inside. You could then "extend" your spikes. However, this could cause some stability problems, depending on how much height your trying to achieve.
Neither of these are ideal fixes, but you were looking for suggestions.
JD