I have a similar problem to you (I have uneven wood floors on second floor), though I do not need to worry about noise. This cheap ($5) solution worked perfectly for me. I went to home depot and bought two round concrete stepping stones that you might ordinarily put in the garden. I also bought some small adhesive rubber feet to put underneath the stepping stones. I put the speakers (w/ spikes) on top of the stepping stones and immediatly got tighter bass response as well as less vibration/sound transferred to the wood floor. This raised the height of my speakers about 2.5 inches, but in my particular speaker (Monitor Audio 5i) this was desirable because they are small in stature and benefit from having the tweeter raised closer to ear level. My only complaint is that there are lots of little holes in the concrete, so sometimes it's hard to place the speakers, because one or more of the spikes slip into the holes. A granite slap, marble slab, or other non-pourous heavy material might do better. Just don't forget the rubber feet to keep the slab of whatever off the floor.