I took a tape I had recorded in S-VHS-ET mode. And inserted it into my parents' old S-VHS vcr. It played-back as standard VHS and looked terrible.
So I drilled a hole in the bottom to trick the old vcr to believe the tape was Super VHS, and I tried again. The picture looked better in SVHS mode, but still badly-distorted.
Apparently the "ET" standard is different from the S-VHS standard, and ET tapes won't play back in old machines that don't have the S-VHS-ET standard.