My first Sony TV was a 20" that I bought about 20 years ago. I, to this day, remember how vivid and bright the picture was on that set. The 20" died a bout 5 years ago and I upgraded to a Sony 32". I remember getting the 32" delivered and setting it up on the same cable outlet that I had my 20" on. I turned on the 32" set for the first time and was shocked by how bad the picture was. I was disgusted by the cable picture on the 32", especially since the 15 year old 20" looked so good on cable.
I though something was wrong with the set until I put a DVD on. Wow, was a difference, the DVD looked great, so for the first time I discovered just how bad a picture cable provides. Some channels looked OK, some were pathetically bad. I called comcast, they came out, made adjustments with their stuff, but still a really bad picture on some channels.
Moral of story, IMHO, the better the TV, the worse the SD PQ is. It's the source that is the problem, not the television