errr...ummm...(cough)...never mind. Problem turned out to be between my ears...
It seems when I built the thing I had labeled the switch in permanent marker on top of the chassis...incorrectly...but had it in the correct position when I biased it. I must have hit the switch accidentally when I swapped the tubes, but thought it was in the right position because the label said so... One of those days.
My range is from about 17-32 mv. I currently have the JJ's biased at around 20. Judging by norton's last comment, sounds like a similar situation.
We now return you to your regularly scheduled program...