Ok, popped off the top and wiggled the tube out of the slot. As Simon predicted, no sound with the valve out on the tube setting (splitting the difference with the terms here ;-) ). Put the valve back in. Sounds the same on both settings.

In the process I learned a couple of things. The published schematics are versions F and K. My board is a G made in 2010. The only difference seems to be an added cap in the DC protection circuit, so that should not effect anything we've been talking about.

I also learned that the tube as a Sovitek 12AX7WA. Not sure if any other tube would make a difference in this slot. Right now it's the only 12AX7 I have ;-)