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.

Thanks, Altec - you must be about my age. One more bit of AR curiosity - do you happen to know if anybody ever tried biamping or even triamping the big AR's? I know it would have been difficult considering the well-sealed enclosure and would have required mounting more binding posts on the rear, but if speaker-level crossovers are a weak point in the reproduction chain someone might have made the attempt.
Just speculating.

Oh, I imagine that somebody probably did use bi or tri amping on the original AR speakers, but I don't know of any instances personally. In those days, people weren't likely to try active crossovers for the simple reason that fewer commercial active crossovers were available. Now we have quite a few, albiet for the PA and musical recording sectors.

The first instance I have read about where bi ampling was used was in a 1951 issue of High Fidelity magazine where it was used in conjunction with the FAS 'bass coupler', an organ-pipe like contraption which must have sounded rather bad by today's standards. The crossover itself was a simple RC passive affair with a tube output buffer.

Altec wrote, [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.] You are so right.

I'll even take that to late 50's recordings available on CD. I have people try to excuse poor sounding 70's classic rock as old recording techniques and then I play them some beautiful 50's recordings to show them that the potential was always there. Sometimes it depends on the Masters still available / used but they definitely knew how to record the material.