I conducted a similar test to compare.
Interesting, within Silverfast, I was/am able to "select" separate color filters for both RGB and Grayscale. However, for each RGB scan with a separate filter for white, red, green and blue--I just didn't see discernable differences. This leads me to wonder that while "selection" is possible--it may not be functionally operational in either Silverfast or may be the 4990 scanner. Hmmmmm!
Also, I compared 16 bit grayscale with filters (white, red, green, blue) and 48 bit scans with similar color separations in P'shop. With the 4990, there are subtle differences--the 48 bit scans with green channel isolation in P'shop generally appear better in terms of detail and noise.
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