I almost never used the RCA audio passthrough on the TBC-1000, I just wired around it unless I was specifically using the distribution amp (which wasn't too often as years went by).
Ah-ha! I told you.
I've gotten pretty good at detecting false sharpening in recent years.
Good skill to have when editing, terrible skill to have when trying to watch TV. (Hard to enjoy something when you notice all the errors.)
I doubt you'll ever be satisfied with composite, especially not under analysis with test patterns. But as long as it;s "good enough", it will serve its purpose as the fall-back connection method, when s-video throughput looks bad or s-video is unavailable.
One trick I've learned is that sometimes -- even on the cheap $5 units -- the svideo>composite adapters can look better than native composite output. It only works in certain situations, but it works nonethless. I use one of those on the office TV, s-video out from DVD player, switchover adapter to composite, into the TV. Entertainment while I work on the no-brainer type projects.
I want to add an Aja Kona or Blackmagic card here, too. So your research is interesting.