Hi theillusionist,
As far as I know, there are THREE AMV Converters out there. All are pretty much loosely based on the same engine to convert AMV Videos.
AMV Converter should convert other formats apart from MPG as I have been able to convert from AVI videos in the past. Check to make sure you've installed all the codecs necessary as it should convert other video formats.
I've also had to edit your comment about getting a registered version of the AMV Converter Studio as we don't promote this on this website.
I haven't used the Bytessence Converter yet unfortunately so can't comment on here. However, it was created by a member on this forum as well so you can ask the question about quality in this post here:
A new AMV Converter program MPxConverter by bytessence.
Hope that helps.