You're welcome, and to all! Glad to share my research and ideas at the forum. And, please, anyone, jump in at any time to add his input.

I'll complete the demonstration with these poor sources, as a worst case scenario. Thereafter, I'd like to finish with the result from your good sources. If you could post/link a matching pair of SD & HD snapshots, of your choice, that would be great!

I can duplicate the paint program's Histogram Adjustment, if I don't use it's Midtone: compress expand (contrast adjustment), with Avisynth's built-in Levels() function. And a quick looks at the Avisynth docs indicates that I should be able to use MergeChroma() or MergeLuma() for HSL split & recombine in a single operation. I'll work out something. :)