The fact that the speakers are amplified and have a 10k input impedance
makes me think you can bridge a bunch of them across a single line and not
worry about loading it much. In broadcast, we used to have 600 ohm sources
and loads (brought over from telephone lines). Inputs were either
terminating (600 ohms) or "bridging" (10k or higher). You could hang a
bunch of bridging loads across the terminated line with no problem. One
other issue is ground loops since both inputs and outputs are unbalanced.
But I don't think even that is an issue if the amplified speakers are
powered by ungrounded wall warts, so the inputs to the amplifiers are
floating.

So, overall, I don't think you need to add any hardware. How about testing
it and seeing?

Harold


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