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For my scan, I go with 1200dpi in PNG, no descreening ( my scanner really doesn't seem to descreen anyway... ) . Then I use Noise Ninja to remove most of the artifact. After, either it's a small Gaussian Blur ( 1px) or a Median Noise ( up to 3px ) if more artefact and finally some sharpening.

I don't do any color manipulation except converting color profile. Some software just doesn't support PNG's gamma correction.