The source of the unused 8th channel issue was the Andrew Marshall Audio Ideas Guide review:
http://www.audio-ideas.com/canadian/axiom-a1400-8-multichannel-power-amplifier.html

He sites a confirmation from Tom Cumberland, the amplifier's designer, but I have never seen any official comment from Axiom.

I run mine without anything connected to the input of the 8th channel without any negative consequences (that I am aware of).