You have to remember that many of these old school covers where originally printed at 72 dpi. Moiré can be caused when downscaling because it's merging pixels; causing patterns. I noticed a bunch of these patterns while downscaling the Batman cover. I tried to use filters but there's only so much one can do. My personally advice is to keep scans between 600 dpi and 300 dpi. For me, it becomes difficult above 600 dpi.