Generally, be very wary of mirroring things. The problem with the traditional Mirror Tool in Max is that is makes a negatively scaled version of the model. You can see this if you select the object and enter this into the MAXScript listener:
You'll get a value of [-1,-1,-1]
The fix for this is to Reset XForm on it.
If you need to use Mirror, use the Mirror Modifier instead of the Transformation Mirroring.