I own the 32" XBR LCD. I subcribe to Time Warner Cable Raleigh which provides digital simulcast of the analogue standard cable package. I also use the Pace Set Top HD Box. This particular box has (I do not know about SA boxes) the ability to set the output resolution to either 1080i, 720p, 480i, 480p, or "PASS".
THE PQ ON SD IS MUCH (AT LEAST 25%) BETTER IN THE "PASS" MODE. Reason being is the scaler in the box is crappy. The scaler in the TV is excellent. In the "pass" mode, the box passes the native resolution of the channel to the TV for processing. HUGE DIFFERENCE.
Again, I don't know if all brands of set-top boxes allow a "pass" through output. If yours doesn't, I encourage you to exchange it for one that does, if available. If you really want to benefit from all the enhanced technology of this set, be aware of what you feed it.