A couple questions back to you. What resolution are the PNG files you are using? What is the dimensions, resolution and format you are exporting? If you are exporting to JPG, how much color reduction are you saving it at. Also are you exporting in Xara and opening as JPG in Photoshop for additional enhancements or compression? Anytime you save a JPG file more than once, the file will become color lossy. I might recommend exporting to some other format, before anything done in PS, then saving as JPG then, only.

Most textures (image fills) that I use are 300 ppi and fairly large images. Often I am printing my maps at high resolution and large format with great results. I also print smaller versions at letter size on my color laser printer, and depending on the level detail - it prints cleanly, no blurring. I also export the same maps at virtual terrain app scale which is anywhere between 50 ppi and 200 ppi depending on the VT app and connection speeds of other players using the client side of VT apps. I have never had complaints.