If you are using Photoshop try sharpening via the Lab Colour method. If you convert your final edit to Lab Colour and then sellect the lightness channel, and use Smart Sharpening. You can switch back to RGB when you have finished the sharpening process. Here's a couple of tutorials:
http://www.dphotojournal.com/photosh...or-sharpening/
http://www.youtube.com/watch?v=OptOm3RkByM
To get round the sharpen/unsharpen bit you will have to produce two files; one for Alamy unsharpened and another sharpened for other stock agencies.