This is a quick tutorial to display a subdivided surface (SubD) with its low-res control in wireframe over the top.
This tutorial will apply to any SubD surface providing you create it by selecting the low-res version and clicking Model>Create>Poly.Mesh>Subdivision not just using the +/- keys to quickly subdivide the original.
Select your model then click Model>Create>Poly.Mesh>Subdivision:
Next, click the palette icon at the bottom left of the display. Now, click on Palette>Display Types>W, this will change the mouse to the picker and means everything you pick will be displayed in wireframe: pick the LOW-RES model by left clicking it, then right click to exit. Note: You can hold the mouse over the other letters to see their function. Finally, in the perspective viewport, uncheck "Override Object Properties"
The low-res version should now be displayed in wireframe with the SubD shaded (depending on your view mode)