Well I think whaat they meant was what amplifiers set new standards that changed the way folks thought about amp design.
At least that is the way i would think if asked the question.
Since i do not know the history of amplifier design, i could not begin to answer the question.
A second way to look at the question would be what amplifiers made the biggest splash in the audiophile world over time from an enthusiasts point of view. And engendered copycat designs.
A third way was what amplifiers have the longest track record, and the greatest enthusiast share of the market. And are still in use by percentage built to percentage still in use by audiophiles.
This last one i can guess at. As it is clear McIntosh is still in there. So is Audio Research, and naturally Bryston.
Adcom might make the top ten here.
Others might be there like Mark Levinson, Krell, Belles, Conrad Johnson, Marantz and many others..
The group which would NOT be there are all those Japanese receivers made in the 1970s... (ecept mysteriously, the Marantz ones..)