Print are always darker than what you see on your screen. The screen is "throwing back" some brightness from its own inner light so when you're making a file, always think that it will be darker by around 15-20%, especially if you use dark colors (though numeric print seems to have slightly less darkening effect than offset print). Though you can't really do anything with it, here's some tips:
- First, work with the same colorimetry parameters as your local printer (ask him which one he uses and set it in PS)
- Second, when you make a file, save a copy, flatten the .psd file, save it as .tiff (best format for print), turn it into RVB, then turn it back to CMJN : this will help to keep closer result to what you have.
For your already made map, you'll have to play with brightness, luminosity and such to find something that fit better, which won't be the best since you already worked on colors :/