When did the Hi-Fi sound mature?

It didn't - it has actually become more immature since the mid 80's. First it started with the "small speaker big sound" and now most Hi-Fi is nothing more than a narrow elegant speaker (made in China) with impressive Boom Boom Tizz (usually with polished metal drivers and phase plugs to make the consumer think "wow technology" versus the tried and true paper/pulp (think "outdated") cones). The reflex port(s) (yes plural as one beer bottle resonance is no longer enough to compete) are unbiquitous as is one note bass.

Not sure who started it but the boombox sound (often supplemented by earth shaking subwoofers) are now demanded by the masses (who are well trained to expect humongous bass and squashed compressed sound): even if it isn't at all musical, it is at least impressive and that's what matters.

I think that Shadorne makes some very valid observations with respect to the equipment that is sold to the majority of consumers (who may have vaguely heard of Bose and believe it to be the ne plus ultra), but the answer is very different with respect to "the average Audiogon system" that you refer to.

With respect to that kind of system, I don't think it's possible to define a clear answer on a system "end to end" basis.

For digital sources I would say that today's average high-end sound began to be approached in the early to mid 90's.

For amplifiers, preamplifiers, turntables, cartridges, and speakers, I would say the early to mid 80's.

For amplifiers and preamplifiers, though, a case could be made for some of the tube designs of the 1960's. And some would even argue that a case could be made for the Western Electric theatre amplifiers of the 1930's (with modifications for home use), if horn speakers (that don't require much power) are being used.

For FM tuners, I would argue 1954, when the REL Precedent was introduced. It was a mono tuner, but usable in stereo in conjunction with separate multiplex adapters that were introduced a few years later.

Regards,
-- Al

It depends.

If you want to play pipe organ or marching band music at volume approaching the live performance a good 1960's system with its 15 or 18 inch woofers will blow away today's tweeky high end system.

If you want to play a female vocalist with "you are there" imaging, today's system is better.