Lotech I have the Panasonic TH-42PV80
I did try the passthrough setting on the HDi box. It didn't make any difference to the clarity of TV 1,2,3 and Prime etc and it stretched out the other channels like Fox, CNN, BBC etc and made them look unnatural.
The conclusion I am reaching is that unless you watch a lot of DVD's, play games or have native HD feeds, you are better off with an old CRT when it comes to picture sharpness.
The stretching of the picture is nothing to do with passthrough. It is to do with what you have specified to happen with 4:3 format both within the HDi box and the TV. I suggest you go through each of the settings carefully and if you don't want stretching set 4:3 to pillarbox.