In dye transfer there were at least 2 methods to increase contrast that immediately come to mind. One is to change the ratios of the A and B parts of the Matrix (tanning) developer to get at least 3 contrast grades if not more, and the other was to change the acid concentration in the post dye bath. I believe that there was a 3rd related to the use of Highlight Reducer.

Anyhow, there ya go!

PE