You're right Gex,
I don't have a Zen amp either, :(
but the way I would do it is from the Zen I would run 2 sets of cables.
One from the Zen to the main speakers and one from the Zen amp to the plate amp high level input. There will be no load change percieved by the Zen amp, but keeping the Zen amp and the main speakers on their own set of cables, as short as possible, will preserve the simplicity of the circuit between the Zen amp and your main speakers.