I was starting to look at the spectrum of the LED
Good point! MGIV has three emulsions since 1984; see:
http://www.photomemorabilia.co.uk/Il...ultigrade.html
and possibly the response of the "medium-hard" emulsion falls in a hole of the LED spectrum. See:, e.g.
http://www.ecse.rpi.edu/~schubert/Li...%20spectra.jpg
http://www.ecse.rpi.edu/~schubert/Li...%20fitting.jpg
Then you are left with only the two extreme emulsions, which makes more evident the inherent problem of variable-contrast paper, when the curves of individual emulsions don't merge well into the equivalent of a single-grade paper.
So much for my (vague) plans to try an LED lamp in my enlarger.