My scanned images look dark, but I think the originals look fine.

It's very possible that your original images actually are dark. Viewing images on a light table or by projecting slides through a projector can be misleading. Blasting a lot of light through a saturated slide may look great. However, the exposure may be questionable. To achieve the best scan possible, it is best to expose your film correctly, according to manufacturer's rating. PURPOSELY OVER-SATURATING OR UNDEREXPOSING POSITIVE FILM MAY NOT BE ADVISABLE. Another reason your scans may look dark may be due to the state of the monitor you're using to view the image. The steps you take to acquire and prepare the image are very important to the outcome of your finished print. Contact your printer for the proper preferences to apply in Photoshop so you can make an accurate separation. Some things to ask about include dot gain, GCR or UCR (type of separation), and gray balance adjustment. Make your own tests to ensure you are preparing your files correctly.