ZERO autotransformers with XA or AX amps?

Has anyone used the ZERO autotransformers with AX or XA amps? If so could you please comment?

I have 4 ohm speakers and as everyone knows this reduces the available output power of these amps. The ZERO's offer a possible solution with the arguable cost of inserting a transformer in the chain.

Has anyone used an XA or AX / ZERO / Magnepan combination?

Thank you.