The Single Piece That Started Today's HIGH END ?

Well, I think we need to define "high-end." If it simply means "high-priced," then I can agree with what Elizabeth and Ferrari have said. However, if there is a correlation to performance or high-fidelity then I have to disagree and say that it started much earlier. Some of the old tube amps with the Klipsch or other high-efficiency speakers still sound pretty impressive even when compared to today's current, high-priced crop. And I realize there have been many refinements over the years in parts quality and driver technology.

I think certain aspects of performance have definitely improved, like lower distortion and lower noise, but some of those early systems did some things so fundamentally right and were exceedingly musical and dynamic...

Linn SONDEK LP12

The early McIntosh and Marantz tube amplifiers, as well as the Klipsch and Altec Lansing horn speakers. How about Quad ESL Loudspeakers?

No doubt Mark Levinson JC-2. It cost about $1000. The AR SP3a which was
considered the best at the time was $350. People thought I was insane
when I bought one.
Drjoe

I was around, and geeked on audio in in early 1970's. I think you have to separate the US, European and Japanese markets as very distinct from each other at that time. In the US, there was an 'old school' consisting of Marantz tube equipment (7c preamp, 9 amp, 10b tuner), some electrostats like the KLH 9 and a few other pieces that, while 'old,' were still considered legendary in the US- by then, horn speakers had pretty much fallen out of fashion for consumer audio.
Levinson's introduction of a very expensive solid state preamp which, if memory served, had a john curl phono section, ARC's introduction of the SP-3, the entry of the Linn, starting the revival of belt drive and, a little later, the dahquist DQ10, which emulated the appearance of the Quad electostat- I remember double-Advents, a cheap speaker but used in combination with the Phase Linear 700 (there was no 'A' at that time) making waves, literally and figuratively. Maggie's came into play at around that time, too- big Typami panels- played louder than the competing electrostats.
My dream in those days was to find a Dukane Ionovac. :)Of course, I was always a little strange. Still had my Phase Linear when I bought my first set of Quads. No, not a synergistic match, but the Dyna Stereo 70 was.

I suggest that "High End" is a design and manufacturing philosophy, more than any particular piece of electronics. Back in the 1930s Stromberg Carlson, among others, built console radio/phonographs that were sonicly greatly superior to most, cosmetically styled, made with the finest workmanship, and, of course, priced accordingly.