speakers on fireplace hearth???

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.