THe 921 output is darker. When I had the 811 and 921 side-by-side PIP you could see a notable difference.
See attached pics: 3 examples 921 left side. 811 right side.
Recalibrating your TV is not the answer. The levels of your TV are constant. To have your set worked on would still net you a dark picture since the technician is actually adjusting the TV from an internal source rather than the 921 output.
This should(can) be addressed by the 921 software, but it seems that most people quit complaining about it since all these other problems cropped up and now the OTA/guide snafu.
If you notice when you first reboot your 921, the screen is about 5% lighter, then it flickers to a darker state right before it gets done and aquires the satellite signal.
The short answer: No workaround other than adjusting your brightness/contrast as a temp fix.