Now, I've changed all the blending modes of each texture layer to Hard Light - this allows the full amount of texture to be projected onto the surface with its full luminosity.

Try different modes like Overlay, or if you want it more subtle, try Soft Light. This is a great way to do Dodging and Burning (without using the Dodge and Burn Tools) and add texture and color, or both, to a fairly basic 3D render.

Doing texturing like this is also easier and have more controls than doing it in the 3D engine unless you have something like BodyPaint, orUVMapper (or "like" tools in say Lightwave, Maya, and 3DS Max to name a few) where you can export the UVs and custom paint them in Photoshop (or Painter, or the program of your choosing).

Hope this helps-
-Lew