Thanks, everyone, for your help.
Okay - I checked the Saunders / LPL D6700 enlarger that I have.
There are no attenuators on this enlarger.
And the lamphouse doesn't have those three position you mention, Trexx.
I had a closer look at the enlarging lens I have - it's a 90mm Roganar-S (I'm waiting to pick up a 6-element lens, but it'll do in a pinch). It stops down from f4 to f22 without a problem.
The thing is, when I compared it to the only other enlarging lens I have (a el-nikkor 50mm), it seemed as if the aperture curtain is off by about a stop. That is, what is f8 on the el-nikkor looks to be a little over f11 on the Rodagon when I eyeball the two curtains. Can two lenses of different lengths be compared this way?
I don't know if a stop difference will be the solution here - but I can't see anything else on the enlarger that would indicate anything else. The lamp is an 85W. It's housed properly. The condenser screen is in place.
It'll be the weekend before I can get back into the darkroom to experiment - but it's a real puzzler for me.
Thanks again for everyone's help.