speakers on fireplace hearth???

Anyone have any experience with placing speakers on bricks in front of unused fireplace? The hearth is raised about 8 inches. Should I place anything under the speakers?...use the spikes? The back of the speakers will be about 10 inches from brick. Should I hang something between the speakers and the brick? Advice or websites that address this issue would be appreciated.
My parents have their moderate sized bookshelf speakers on a travertine fireplace hearth bench about a foot and a half off the floor. They are using BDR cones under each speaker. No problems, sounds great.
Are the speakers bi-polar, di-polar, or have a significant backwave (ported)? If so you may have issues with that large resonating chamber called a chiminey ;<{) Consider hanging something over the fireplace if that is an issue.
These are front port bipolar speakers? The fireplace has folding glass doors. What would you suggest that I hang???
Should I use the spikes??
I just finished reconfiguring my room because of the hearth problem. For 8 years I've had one speaker up on the hearth and one on the floor. I got new speakers and decided to re-wire everything so that I could get the right speaker placement and I am so glad that I did. The issue I had was no flexibility in terms of placement. The bigger problem now is the WAF, which is quite low at the moment. Oh well.
I have a similar situation with ML SL3s surrounding a fireplace that has a TV in it - another large glass area that bounces sound. I have a wool numdah (wall hanging) that I hang over the fireplace when I'm not using the TV. My advice is to try something cheap and quick and see if it makes a difference for you and then take it from there.