Try: View menu>Disply Performance>High Quality Display
It may be your display settings. For hi-res preview set View > Display Performance > High Quality Display (and you can set this as the default in the prefs with no files open).
You should also check the "Effective PPI" in the info panel to be sure the number is high enough for your print process. In general there's little chance things will improve by upsampling in Photoshop.
You should also consider the size of the image (pixels) relative to its print size.
desired print resolution: 300dpi
desired image print dimension: 8.5" wide
necessary pixel width: 8.5" x 300dpi = 2550px
For a 'background' image, you may be satisfied with 150-200dpi, so the 8.5"-wide image could be as small as 1275-1700px wide.
That's what the effective ppi number is all about....
Hey, that is helpful. (I glossed over that part of your reply.) Learn something new every day!