I would really question putting the speakers so close to the fireplace with the glass. If you convert back to a wire screen, you’re definitely asking for trouble. 1.5ft is nothing even though they’re off to the side. A decent fire puts out a lot of heat. And, it absolutely doesn’t all go up the flue. Add the radiant heat factor & you’ll toast those speakers over time. If you doubt how much heat they’ll be exposed to sit 1.5ft from the fireplace... with a really good fire. It sounds like you don’t have it but, I would never put speakers less than 5ft from a fireplace.
Just when we have all the technical electrical theory things balanced out, the tonearm adjusted & a listening sweet spot figured out... the real world laughs.