The classic AR speakers were not like today's in that they did not have a tipped-up "tizzy" high end. If anything, they had a rather mellow extreme high end, which lent itself to sounding very convincingly like real musical instruments. An overly accentuated treble makes a speaker sound more like "HiFi" than real musicians, and is a sound I personally do not like. The upper midrange was a bit laid back however, and this gives the impression of more "depth", for better or worse.

The AR live vs recorded demos were recorded on analog 1/4" tape in anechoic conditions out-of-doors so that there was not any built in "room sound". This let the sound of the hall in which the music was played furnish the ambiance, which was the exact same ambiance supplied to the live musicians. The effect was convincing, especially because the audience was far enough away that the spatial relationships were difficult to tell apart between the musicians and the speakers.

Yes, Dynaco MK-III 60 watt amplifiers were used for these demos. This was in the days before AR came out with their own 50 watt "AR Amplifier".

Ah, progress?? Interesting you mention that. I recently posted a thread called "Smackdown - have recordings improved over fifty years?" in the Sound and Vision forums. My intention was to take vinyl reissues of classic jazz recordings made around 1958 - the sound that the proverbial audiophile of 1958 would achieve - and compare it with the latest and greatest digital jazz recordings made now - what that audiophile would hear today. I interspersed the vinyl selections with the digital selections on a CD so that it could be easily distributed.

Interestingly (but not surprising to me), the 1958 vintage recordings on vinyl were generally acknowledged as sounding more like real musicians playing in front of the listener than the 2008 digital recordings.

Progress???? I think they already had it right in the 1950s - all that has taken place since has been marketing department hype.