The reason KNXV switches to SD for the end is so they can squeeze the credits to lay on their promo junk for the 10PM news.
The reason the sound is tinny for some and not others is that the dialog in a real 5.1 setup is supposed to come from the center channel only, with the front right and left speakers used mainly for directional effects. However, KNXV mixes the center channel out to all 3 front speakers. This is no problem for a 5.1 system, because it decodes correctly. However, when downmixing to 2.0, the mix of center+left+right to left/right basically duplicates the center channel with a few ms delay, in addition to what was already on the left right, creating a nice echo/tinny sound. Anyone with a PC HD capture card and 2.0 speakers or headphones can tell this quite easily. Switch your AC3 decoder's output preference to 5.1, and the echoes go away since you only hear what is on the front left and front right channels in the AC3, but switch it to 2.0 and you hear everything mixed down to 2.0, including a delayed center channel.