Cowanw, you ask an important question. My understanding of the workings of the VC papers, especially Ilford ones, and very much MGIV WT, which similar to Bob, I like very much, is that it should not make any difference, whether you use blue and green, or a magenta and yellow filters. The paper's emulsions primarily respond to blue and green spectra of the white light. A magenta filter is blue with some red added, to which the paper is not sensitive (within the bounds of normal duration exposures), but which makes the image brighter for the worker. Similarly, the yellow filter, is simply green plus the comfort-enhancing red.
Originally Posted by cowanw
I would hypothesise that if you used a colour head, by means of which you mix the colour channels, to match the Ilford lowest two grades—contrast range ISO (R) 150–170—you would experience the same wobbliness of the paper's characteristic curve. I do not have such a light source, so I cannot run the test. Even better, if someone else did it, to verify this hypothesis.
I think that is the beauty of what we do, there are many ways to arrive at a wonderful result on paper. To some a Les McLean split-grade is the key, as it seems to have helped the OP, to others, like Bob Carnie, MFM, yet others, like John Sexton or Michael 1974, prefer the more old-fashioned approach of building up the grade in small steps. The real jewel is not the knowledge of a general technique, but its very precise detail, such as described so patiently by Bob. These things are not obvious, such as Bob's grade 5 final blast, or like the minor extension of the base time exposure, needed when using the old-fashioned approach while climbing grades, to maintain the highlight density—which split-grade approach does differently.
Originally Posted by Bob Carnie
Personally, I couldn't figure out my grades without split-grade printing a few years ago, and I have been thankful for that technique. Now I prefer the old-fashioned approach. They both work, and they generate an amazing amount of enjoyable reading on APUG, thanks to which, I get ideas for improving my printing.