Great observations here.
The tube IFs of Zeniths using the 6EJ7-6EH7 were higher gain than RCA's tube IF but the Zenith SS starting w/1970 seemed to up the whole game. Experience does bear this out:
My grandparents had a 20X1C38 on an antenna in S. FLA and my fuzzy memory recalls it was a tad bit grainy oven on strong channels. But the Zenith's brightness, focus and stable HV hid that fact somewhat.
I had a 1969 Zenith (16Z8C50) on Storer cable in Dallas and the picture was not as good as some SS-IF strip Zenith sets probably for this reason. Its easier to notice grainy HF video and snow on a 23" set versus a smaller set too
