I appreciate the replies. Here's some more detail and what I've tried so far...
Friends came over for movie night and the sub sits right below my screen so I always notice the green light pop on as things are powering on, but not so this night. I checked the receiver first because once I had it not "see" the subwoofer and stop sending the line out to the sub (there's a little icon labeled sub on the front of the receiver and it was not lit.) But that was easily remedied by going into the manual setup and re-enabling the sub. Last night the sub light was lit on the receiver and when I turned on the test tones, it appears to send to the subwoofer channel, but no sound comes out of the sub and the active/standby light on the EP-350 stays red.
After checking the receiver, I tried a new cable and also flipping the auto on/off switch to "Activate". No luck, so I tried running a line out from my iPhone to the line in on the sub to see if the sub port on the receiver was bad. Still wouldn't activate.
I checked the knobs on the back of the 350 and they all looked like they were set correctly. Tried unplugging power for a few hours and switching the phase back and forth. No go.
I found a couple other posts similar to mine and it sounds like I need to speak to customer service and do some troubleshooting. I'm still within the warranty period so I'm sure they'll take good care of me.
I will say that I have always had the Axioms setup as 5.1 and had never heard my setup in 5.0 mode and I was blown away by how well the system handled Pacific Rim without the sub. The M-22's have some real punch when run in large mode.