I felt a bit disappointed after the charchoal trial when the full spectrum of values was not there. After doing some thinking I suddenly remembered Igor Kazarin, http://www.art-portrets.ru/dry_brush_technique.html and his drybrush technique. He is able to produce wonderful portraits with this technique very close to photorealism.
It is a promising technique if you want to be able to get the really dark values as well as the light ones. I used Old Holland Mars black for this trial on Montval watercolor paper. It should be interesting to try out a few different blacks to go with certain watercolor paper. Igor mentions using Fabriano watercolor paper along with the black oil color Tiondigo that unfortunately is out of production.