The Single Piece That Started Today's HIGH END ?

Well I have been in this hobby/business since 1957,when all its was was tubes and mono and about everything was kit built. In my opinion and speaking of high end as to its real start was about 1975 or 1976 with Threshold, Levinson, Krell and CM Labs. The first of the true mil-spec products,and a build quality that had not been seen before. These products finally delivered not only the promise of high end, but redefined audio components in America and were American built. Many of these early units from these companies remain in service today some 25 to 30 years since they were new and continue to deliver the promise. Prior to the debut of these, most of the gear,just did not have the sonics or build quality of these units. This was a major quantum leap in audio and remains so since the mid 70s.

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.