Well, I looked at things with a fresh set of eyes today. I roped in some of my colleagues in the darkroom as well. My conclusions were the same; the prints focused by eye were sharper. You really can't miss it on the test negatives, because of the series of fine lines, swirls, etc... I should scan these so people can see what I'm talking about. But it's quite apparent. The test negs are rather brilliant for that; errors in focus stick out like a sore thumb.
I then focused the test neg by eye, noting the column height. The grain focusers were focusing roughly 2mm closer to the neg than I feel was correct. The durst column has these markings, so it's not too hard to see. Another thing I did was re-focus by eye, then go through the whole range of adjustments (from lowest setting to highest - it's helical focus) of the grain focuser eyepiece. Never did it look visually sharp. Not that critical sharpness I see when I move the lens 2mm closer to the neg.
So, what Bob-D659 says is relevant. Maybe all these grain focusers have been dropped, or had glass replaced, etc... Who knows. It's all old equipment. None of it was bought new, I don't think. I doubt it, personally, but it is possible.
I did a quick and dirty test to see where the base of the grain focuser had to be in order for it to jive with my focusing by eye. It looked like it needed to be 5mm to 10mm higher off the baseboard. I loathe the idea of shimming a grain focuser.
Perhaps the more mathematically sound members can determine weather this approximate 2mm difference will be apparent visually when making a print, stopped down to a working aperture.
My feelings are why go to the trouble of using a glass carrier, aligning my enlarger, etc... if my grain focuser is slightly off?
Sometime this week I reckon I'll get my eyes checked to see if I've got astigmatism or whatever it's called. Maybe I'll post a follow up with those results!
Oh, and tkamiya: I'm sure that the focus column is not slipping. The Durst 138s is a beautiful machine. I routinely leave a negative in the enlarger for a weekend, and come back to make more prints a few days later and the grain is still as sharp as when I left it. I believe I can almost 100% rule that out of the equation.
Thanks for all your thoughts on the matter.