The 73640 is one of the newer models, all of the end digits of 40 seem to be the most recent models.
Per the Onwers Guide page 22 it seems that "Depth" adjusts the depth in the "2D + Depth" (2D to 3D conversion) feature. It does not sound like it is intended for real 3D signals.
2D+ Depth When selected, generates a simulated
3D image from an ordinary
2D image. Adjust the effect using
the Depth setting. Requires use of
active-shutter 3D glasses.
If your depth seems incorrect with real 3D signals, I suggest you change try the "Glasses L-R" in the opposite setting to see if that improves the image.