Most of my video production

Most of my video production uses archive material, so I have SD at 768 x 576 and then HD 1980 x 1080 from the current cameras. Using an HD 720 preset works for me fine.

When you say distortion, I presume you mean that circles aren’t circular but oval? If on the other hand you just mean circles are circles but just soft or fuzzy edged, then it’s just a lack of pixels. Between 768 x 576 and 600 x 480 the pixel count is nearly halved! I think your problem is that 600 x 480 is not even SD definition so you’re losing quality all over the place. 1280 x 720 works well for me from decent source.