inverted shelving brackets?

[Mike Johnson]

It sounds like a good place to use a floating shelf. They're not hard to find, or build yourself. If you want a suspended shelf, you could use wire or cable. They're harder to find ready-made, but easy to piece together. Or, hide an angle or L-bracket behind the speaker.

As long as the shelf is the right size for the speaker, nobody will be tempted to stack lead sash weights on it or what have you.

So much work just to avoid couple tiny screw holes in the back of the speaker cabinet!