Try Quadtone RIP for the monochrome pano. Are you running Cone or MIS inks (or a mixture of them), or are you dithering using colour inks?
Take a look at Kernow's selection of inkjet canvas (the Kernewek fabrics). I'm not a fan of most canvases, but I saw these ones a few weeks ago, and they're quite different. Unlike most of the others, they're not primed - there's no layer of crumbly, chalky material covering the fabric - and they seem to produce deep blacks for a canvas. Of course, coating them with a gloss spray would enhance the blacks even more. I'm considering getting some myself for colour and black-and-white printing.