This machine had absolutely terrible video. I tried all the mods like supervideo and it was still really bad. My 400s rf was much better! lol Anyway, couple years ago I bought the lowbudget board off ebay. It takes it's signals direct off the gtia and was pretty decent. Still had some lines though so thought I'd give your board a try. Clears things up even more. I'm leaning to putting my 2nd board in the 400. It was my original Atari and I only have 1 tv left that uses RF.
Thanks for your work on this!