It's digital. How good does it need to be?
Shouldn't be any difference in picture quality from one tuner to another.
Maybe sensitivity and error rate though.
Or is there a difference in what they are capable of tuning in to?
There is a FAQ or thread around here somewhere that details the chips used in the S3 and new TiVo HD and even has pics of the mainboard and components.
If I'm paranoid, it's only because everyone is against me.