Here is another couple of possible explanations:
1. The Sony manual on page 77 state the resolution choices at the highest levels include 1280/1024 and 1920x1080.
It does not state it can support 1680x 1050.
2. The highest resolution input of 1920/1080 requires the H sync =67.5kHz and V=68 Hz.
Perhaps your Dell is not set to output these sync frequencies and so the XBR2 will not accept your Hi Res signal.
Question - why does the sony manual say that a 1920 X 1080 pc output will NOT fill the entire screen?