It was the done thing to make hi-fi speakers as heavy as possible as this apparently increased the highness of the fidelity. I remember a DIY hi-fi speaker in the 70s (in Practical Electronics or Wireless World, I think) which had double-skinned walls. After putting together the base, sides, and front/back, and putting the speakers exactly where you wanted them, the walls were then filled with sand and the top put on.