Renderman eval and displacement maps

[system]

Hi to all… I’ve got some problems with renderman for maya 3.0 and zbrush displacement.

I follow this workflow:

1. detail in zbrush 3.5 and export the lowest subdivision
2. create and exported a 4096 displacement, flipped, 32 bit, adaptive.
3. import in maya the obj (converted in a subdivision surface), create a blinn, apply a displacement shader, setting alpha offset and gain to 1 and -0.5 and render it with Renderman

This is the result:
http://img17.imageshack.us/img17/7351/pumpkind.jpg

Under the quality setting in renderman render option I set the shading rate to the lowest value (0.1). The problem is less visible but still present.
Any suggestion?

Thanks in advance!