HEY CHRIS, try this. Scan your cover(s) to1200dpi .tiff. they won't be lossless but they'll be huge. That's ok you aint gonna keep it! Build your .jpg's off of that monster.
I'll be you retain higher quality!!
Actually, I did scan in a few at 1200dpi, GIMPs XCF format (Nothing like a 200MB image hanging around). There just was not any improvement that I could find. 300dpi to 600dpi, yes, there is an improvement for sure.
I've been uploading a bunch of R1 high quality Blu-ray/4K/3D movies and TV shows over the years. Scanned and finished (Using GIMP) at 600DPI though as I like the higher quality that it provides. I too would like to see more high quality scans. Most of the scans quality are not so good. On the rare occasion that I download a cover, most times I'm having to put the image into GIMP to repair, remove defects, remove the moire, etc.