Hi Pilou,

Very interesting experiment, but not so clear...

Could you please indicate the precise order of the steps you followed ?
If possible, could you post that obj file ?

Usually when i want to play with Max's subDiv, i import low-poly obj meshes,
in order to obtain a smoother nurbs solid or surfaces.

Another workflow that i use in order to get a smoother nurbs from obj is :

1) Import hi-poly obj in 3DCoat and perform and AUTOPO (auto retopology) in order to obatin a low-poly quad mesh
For this step, you can use any other retopology tool, like for example the very good "Instant Meshes"
(http://www.cgchannel.com/2015/11/download-free-auto-retopology-tool-instant-meshes/)

2) Export the low-poly quad obj
3) Import the low-poly into Moi
4) Run Max'subDiv

If i'm not satisfied with the resulting nurbs, then in Moi i follow a manual "reverse engineering" of the nurbs object,
that is I use the nurbs object just as a reference :

1) Slice the nurbs by Projecting straight lines or curves onto it, using the "Project" command with "Mode : Direction"
2) Tune-up the resulting curves by tweaking control points
3) Using the curves with Loft/Sweep/Network in order to recreate the various areas of the original object
4) Blending the different areas

Obviously using this workflow, the final nurbs object differs more or less from the original nurbs object obtained by first step (Max's subDiv),
but when the precision is not a crucial point, in this way i get a final nurbs object made by a much lower amount of control points,
and made by bigger surface patches.

Nice night!

- Marco (mkdm)