Thanks, Tom. Of course, the B+W is not a cheap filter (unless you consider $70 cheap) but I too have been shooting for over 50 years, and so have memories of earlier glass guiding my expectations.
In short: I kept protective filters on all my lenses, and of course most of those 50 were shooting on film.
As this discussion goes on, I think I'm coming to realize a couple of things:
1) that the lenses of the time were more scratch-prone, and 2) were used with film, and 3) probably were not quite up to the same capabilities as today's glass (please, no flames: I loved my 105! ) and 4) my color images were printed by a lab and 5) I never looked at a negative as closely as I'm looking at NEFs, and 6) as noted above, I never tried the experiment until today.
so it's entirely likely that I simply could not see the difference with or without (other than the effect of UV, of course)... and so today's experiment came as a surprise.
Certainly there's almost no point in "dumbing down" the resolution possible with a 24 meg sensor, so I'll just have to adjust to a bare front element. (Naturally, I'll keep the filter for adverse conditions...)
Thanks to one and all for the comments so far. (More are welcome!)