What Software are you using to print JPEG or other Image based Filetypes?
I implemented Qimage and Adobe Photoshop CS4 (rather old version but the licensing is cheap in comparision to the CreativeCloud version) to print on varios Xerox machines and other 17 x 24" printers we have.
egg. Qimage will prerender any image to the native printer resolution by blowing the resolution up to the specific value.
professional printing is not just simply pressing buttons, there is always a workflow behind it and one must follow it to get reproducable results.
I guess even Xerox has some tools up in there sleve, maybe give them a call and ask them.