So far as I can tell, there are no settings on the player itself that I can tweak... now I'm playing around with the AMV converter program to see if I can get a resolution which works with my player. Seems the resolution at which I converted the videos was probably too high for my particular player, so I'll try something simpler.
I mostly use the Nano clone for listening to podcasts anyway, so if all else fails and I cannot make the videos play, I don't really care, since I have a Rock Chip-based player on order from MP4 Nation.