I'm assuming that all you've changed in your theatre is the screen.
Have you tried running a direct composite (Y/R/W) lead from one of your devices (DVD or Tivo) into the composite inputs on the Bravia? Does anything come up when you don't hook it through the receiver? Just trying to work out with this test whether the receiver's turning a simple output into something else, or if the receiver isn't giving out a signal on the composite leads any more.
Oh, and please try the different Y/R/W inputs too. Maybe one of them is faulty? I might be wrong, but I think you have to set which of the AV/Composite inputs you're using through the Sony menus. From memory, most of the XBR9 series have just two (one on the side, and one at the rear).
It would be interesting to see if a direct component (R/G/B) signal works, as this would give you a higher picture quality overall, and from the AVR-1604 manual I found online, it would allow you to switch between two RGB sources.
Try one of your devices straight into the screen via Y/R/W leads, and if that's not working, you'll need to get into the setup/settings menu on the Bravia and try to work out what's going wrong there.
Best of luck with it, and I think you'll love that screen once you get it working.
Here's the set up that worked before I introduced a new Sony Bravia (XBR9 series to the mix). Now I cannot get a picture on the TV, though I can still get sound (which will be obvious once you see the set up).
Tivo + DVR --via red/white/yellow cables--> Denon AVR 1604 (an older, nondigital a/v receiver)
DVD --via red/white/yellow cables--> receiver
receiver --via yellow video cable --> TV.
To me, that seemed simple. I have tried both a regular yellow cable and a composite video cable (red, blue, green) into the Video 1 on the back of the Bravia. No picture, at all; not even with the DVD player.
The Bravia's external inputs seem correctly set up.
Any ideas? Is there a channel that I need to turn the tv to, kind of like with the old tv I'd set the switch on the back to channel 3 and then tune to 3 to make the thing act as a dummy monitor?
I'm on the verge of sending this fancy new tv back before I throw it through a window. So, any help appreciated!