Ironsoul, you might have hit on something.  I tried experimenting with using the alternate diffuse just now.  Here's what I did.

I applied the brush bristle texture map to both the diffuse and alternate diffuse nodes.  I reduced the strength of the basic diffuse down (this will need adjustment to balance the colors correctly but I was just experimenting here).

I left the handle its default basic color.  No maps used (just a simple MAT file with no extra nodes from Poser for a basic shiny green)

Then rendered.  Notice what has happened.  The brush portion has rendered somewhat correctly but lacks any shadowing, the handle is still in shadow.  However we still have the box shadow.  Also the after experimenting it seems the shadowing on the object isn't being applied so it comes out flat and almost monochromatic.

Not sure what this tells us other than Poser has difficulty rendering shadows for some small objects using depth shadows and normal diffuse textures, but not alternate diffuse.

I would really like to figure out why Laurie isn't having this problem.  Thanks for the info Laurie, sounds like we are both modeling in a simlar fashion (and for much the same reasons), with similar detail in vertices.  The above brush has 1002 vertices and 1020 quad polys.  The only place in the mesh that I would say is "dense" is on the handle where you see the sculpted bits, there are some loops of vertices there fairly close together for the detail.