Here's a great formula given to me by Chad Jarvis (www.redhillphoto.com) for bringing a not quite bright enough kallitype back to life. The highlights lighten up well but what really impressed me is how the mid-tones opened up.
10ml of a 10% working solution of Farmers Reducer
25ml of hypo (same as for when printing the kallitypes)
1 litre water
5-10 minutes in this is usually sufficient.