I just received my X1600 unit yesterday afternoon. I didn't get around to testing it out until this morning.
I have the X1600 plugged into a wall outlet, not the Vagabond II. As I understand, the modeling lamp should not be used when plugged into the Vagabond II.
The modeling lamp is depressed (ON) and set to 100%. The range switch is released to show that the modeling lamp switch is depressed (comparison). The ready light is green (not shown in the picture because the unit is powered off).The bulb is screwed in all the way. Here is a picture of the settings:

Everything else seems to be working fine.
If I don't find a solution, then I will call PCB when they are open on Monday.
Edit: I've taken out the modeling lamp bulb and placed it in my normal house lamp (from IKEA) and the bulb works fine; so it's definitely not a bad/burnt bulb.