@D'@ve - It's less important, but bass frequencies can have lots of energy that will transfer through solid objects. More mass generally means a greater dampening effect on vibration (i.e. sound). Nevertheless, you'll create a 'muddy picture' of where instruments are coming from in the sound stage without isolation. Using spikes will massively reduce the resonance transfer into the floor, sharpening up that image.
They are a pain, they can be dangerous, but learn to love them.
Another couple of top tips (
@Ed Selley: I hope there's more of this type of thing to come because there are so many sub-topics on this to expand into to get a good sound):
1) Don't rock the stand/speaker. From rear left corner to front right corner, try and move the speaker back and forth. Do the same with the opposite corners. Does the speaker rock? That's not good! It will move and vibrate in an undesirable fashion. Adjust the spike height to keep the speaker level and free from rock. Generally you will find that adjusting the opposite corner spike will eliminate the problem as the rock is caused by the other corner spikes being too high. I start off by rotating them all the way in to maximise the amount of thread in the pedestal for the most stability, then adjust to perfect. This can take a long time; it really is worth it.
2) Don't wonder outside the triangle. Ed talked about this triangle, the sides of which are the imaginary lines between each stereo speaker and the sweet spot/listening position. If the side between the speaker is longer than the other two sides, there will be a gaping hole in the middle of the sound stage. The effect will be like two singers each side of centre, or two identical instruments, rather than one anchored to the centre. I know it feels like you might be crushing or narrowing the sound stage by reducing the length of the speakers' side, but getting these distances right is pretty critical. If you haven't already done so, try breaking the rule. Simply walk half way up to towards the speakers and crouch. Can you make out the hole? That's what you're looking to avoid.
Cheers