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.
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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.