Digital BW, The Print

Nick,

One last thing to look for that I don't think has been mentioned 
directly is a gamma mismatch between the Gray Space and the RGB 
Space. Dan Culbertson posted a good method to make the transition 
from Gray to RGB if you are unsure if the gamma's match or not.

"2.Convert it to RGB.(note -be sure to convert to an RGB space 
identical in gamma to the source grayscale space. If you are not sure 
then convert to multichannel, make two duplicates of the grayscale 
channel, then convert to RGB. This will sidestep any gamma change 
when converting to RGB)."

Check you image histogram before and after the gray to RGB 
conversion. If they are not the same, the gamma's don't match.

Good Luck,

Martin Wesley