I haven't seen any improvement in SD from either an 811 or 921 vs a 4900 or 510 standard definition receiver. In fact, on both of my TVs, the SD quality from my 811 is better when I use the s-video output from the 811 vs the DVI-D output.
So when I watch HD, I use my set's HDMI inputs, or component. But when I watch SD, I switch over to the s-video connection. Significantly cleaner picture. Several others have reported the same findings here in many different threads on the 811.
To the original poster, if you are using the s-video output from your receiver, then it will be essentially the same as any Dish receiver's s-video output. I've owned the following Dish receivers: 4000, Dishplayer 7100, 4700, 4900, 510, and 811, and I've tested a 921, and the s-video outputs on SD channels were essentially the same on all of them.
If by "Sony XBR40" you mean the large Sony 40" CRT set, well, images on that set weren't all that sharp to begin with. Sony had a number of problems getting the geometry correct (they never did) and I always found that picture to be a bit soft. Sony discontinued it last year. If you are using the coax/RF connector or the composite video (RCA jack) to connect the receiver to the Sony, then those do degrade the image. You definitely want to be using the s-video connector. If your receiver is a very old one that doesn't have an s-video out, then picking up a 301 or a used 4900 would be a good idea.
Or see if you can get a low-cost lease on the 811. If you live near a city, odds are that you can pick up some HD locals by using an antenna and then you'll get a much better picture on the Sony. And consider picking up the HD pack from Dish, they have been offering it at half-price for a year for new HD subs.
