are you sure its not a units & scale factor..
I work in meters real world cooridnates and alot of the architects that supply me with data work in mm but set their autocad to unitless or just units so when I attach the dwg as meters or mm it always lands in the wrong place... so have to change the scale ref attachment either x1000 or divide 1000...
why not try creating a series of known coordiante points in a dgn and turn it into DWG and supply it as default template file for them then when you get it back from them and ref attach this to your original pre converted file and see what happens and then trial and error the scale up or down...
other wise ask your auto cad guy to draw you some circles at Diaimeter 10 and at some know coordiantes then to send you the file and you measure those coordiantes without conversion and see what happens....then try ref attaching it...