I found a best solution.
I export the page as PDF then, in Photoshop, I import this file and replace the illustration with the original TIFF (putting it with the right resolution, size and position). Finally I export the page as PDF. The page so created shows the image like the original, without the too evident banding, both in Adobe Acrobat Reader than in others viewers. I did a test. I exported the full document as PDF but I replaced the "faulty" page with the adjusted one using a software to modify PDF documents. The result was ok.
It's a pity that Scribus gives me this trouble, even if , how it has been demonstrated by mnawij, this could be a false problem: a thing is what is viewed in the screen, another is what you get printed.
Thank you all for the help.