The Reagan era, and unreported /unreportable (illegal) income no doubt fed at least part of the high end market.
Otherwise, there's always been a high end market, Web search "orchestrons", and "residence pipe organs".
There may have always been a high end market but the OP was about amplifiers not orchstrons. I think this is about typical stereo type audio gear. My theory is that there the spread of price has broadened. There's more than one
$600,000 amp available now. That's around 11 times median HHI. I am pretty sure there are dozens of amps at household income (HHI). Were there any $6000 amps in 1965 priced at or close to HHI.
Anther way of looking at this would be to index amplifier price to home price. The average house in the united states sells for $309,000 the most expensive amp is $650,000. Lets say that high end starts at 25% of median home price or about $75,000. Maybe that's a bit too high, so say high end is 10% of median home price or $31,000. Above 50% average home price is ultra high end. I like this measure better because it's kind of a measure of purchasing power for consumers.
According to the US census:
Median home price is 1965 was $21,000 so a $2,100 amp would qualify.
Median home price is 1975 was $37,900 so a $3,790 amp would qualify.
Median home price is 1985 was $80,500 so a $8,500 amp would qualify.
Median home price is 1995 was $133,700 so a $13,370 amp would qualify.
Median home price is 2005 was $226,100 so a $22,610 amp would qualify.
Median home price is 2015 was $255,100 so a $25,100 amp would qualify.
Most
Macintosh products will not make this cut but there are plenty of amps that will now. So, Mac is really more like a premium, say BMW 5 series kind of product but not so much like a Ferrari or Lamborghini.