The location of the fireplace sounds good. Is it brick or pre-fab? If pre-fab you could get some really bad rattles--but you would have rejected the idea by now if that were the case. If it's brick--the brick isn't going to vibrate much--so any insulation is not needed unless the sub is rattling. The only concern might be the damper and the flue. I would consider closing it off and possibly putting in a heavy sheet of plywood layered with Owens Corning on either side. You could even go to the extent of using some small leveling jacks to push the sub down (put a sheet of plywood on it too) and push the plywood up into the damper. This would really make for a "sound" connection. The fireplace likely improves the efficiency of the sub--no loss of energy (or very very little) through it--so it's all directed into the room. If the fireplace is less than 1/4 wavelength of the crossover point (4.7 feet for 60 Hz) then I can't imagine it causing any strange anamolies in the frequency spectrum. If it's greater than 1/4 wavelength you might consider measuring it's response in and out of the fireplace just as a precaution. But it sounds like it's small.
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