When you unplug the 600, the impedance that your Denon is seeing goes down. Hence its output goes up and that's why you are seeing the spike. The 600's input impedance is sucking down your Denon's output...just like it's sucking down mine. Now don't ask me what design decisions lead to a low input impedance. Maybe Axiom was worried about noise...I don't know.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated