I've converted avi to MPEG2 for quite awhile using avsynth and Henc, but now I'm playing with MeGui, learning to convert to mkv. The clip I'm using has 5.1 ch. AC3 for audio. When I use the One-Click option, the audio is kept as 5.1; but then I can't use avisynth filters. If I select to use an avisynth script or select the avi directly for input, the audio always gets downmixed to 2 ch. stereo. I have the audio settings set to Aften ac3 scratchpad, and Keep Original Channels selected. Is there any way to keep 5.1? Ideally, I guess it should just pass through. Any ideas what am I doing wrong, or can this even be done this way?

Also, dumb question of the day. Something I've always wondered about. x264 is supposed to be "the thing" for compressing to a
smaller output size. But if I have, say, a 1 gb avi file and the final size can be as big or even bigger, would mpeg2 or other codec give just as good quality?