Okay I tried a few things.
I switched patch cables just in case this one was faulty.
I set the subwoofer volume control to about 9 o'clock (low)
I set the amplifier's subwoofer output level to +10 db (!!)

The sub light still only goes from amber to green on loud explosions or other heavy bass notes.

Also, in some cases the light goes from amber back to green in like 10 seconds. Is that normal?

I guess I will send a detailed email to Axiom tech support.

M60, QS8, VP100 Yamaha RX-V2600