Subjective non-blind evaluation methods, based on listener training, careful listening, and documentation, were preferred at the inception of stereophonic sound by its inventor, Dr. Harvey Fletcher, and by other scientists at Bell Telephone Laboratories, the General Electric Corporation (GE), the Radio Corporation of America (RCA), the British Broadcasting Corporation (BBC) and by scientists at many other reputable research organizations who worked on improvements to stereophonic sound equipment.
This article provides an overview of the subjective stereophonic sound evaluation methods from the 1930's to the 1960's. The term "stereophonic" did not enter the literature until it was introduced in 1927 by Bell Laboratories (the research and development division of the Western Electric Corporation). Up to then, multichannel audio systems were known as "auditory perspective" systems. After 1927, the terms "auditory perspective" and "stereophonic" were used interchangeably in the scientific literature. "Stereophonic", or "stereo" for short, can correctly be applied to any multichannel audio system using two or more speakers where the intent is to generate a three dimensional sound stage.
The basic definition of stereophonic is: "of or having to do with a system of sound reproduction in which the sound reaching each of two or more microphones placed apart is reproduced by one of a corresponding number of loudspeakers, also placed apart, giving an effect of depth and direction much like that of the original sound." 
The word stereophonic is derived from the Greek words "stereos" meaning "hard" or "solid" and "phone" meaning "sound" or "voice". Literally, a stereophonic system is one which creates an illusion of "solid sound".
The first documented experiment in multichannel audio systems was conducted in 1881 by Clement Adler. Adam Blumlein invented and patented a two channel (binaural) audio system using headphones in 1933. Dr. Harvey Fletcher, a contemporary of Blumlein, led the research team at Bell Laboratories that developed the modern concept of the "home stereo" system. The original prototypes were three speaker systems. This was revised down to a two channel, two loudspeaker system that was less cumbersome, and therefore more acceptable, to consumers. This work lead to Dr. Fletcher being honored as "the father of stereophonic sound".
In a technical paper presented at the Winter Convention of the American Institute of Electrical Engineers* in New York City in 1934 (January 23-26), Dr. Fletcher stated:
"This symposium describes principles and apparatus involved in the reproduction of music in large halls, the reproduction being of such a character that may give even greater emotional thrills to music lovers than those experienced from the original music. This statement is based upon the testimony of those who have heard some of the few concerts reproduced by the apparatus which will be described in the papers of this symposium. [Emphasis mine.]
The most intense peaks in music come in the range between 200 and 1000 cycles per second. Taking an average for this range it may be seen that there is approximately a 100-dB range in intensity for the music, provided about 10 dB is allowed for the masking of sound in the concert hall even when the audience is quietest.
The music from the largest orchestra utilizes only 70 dB of this range when it plays in a concert hall of usual size.
For halls like the Academy of Music in Philadelphia and Carnegie Hall in New York City,...,the power of the sound source is approximately 400 watts.
A person would experience the sense of feeling when closer than about 10 meters to such a source of 400 watts power, even in free open space. [An emphasis on the word "feeling" appears in the original text.]
These then, are the general fundamental requirements for an ideal transmission [playback] system. How near they can be realized with apparatus that we now know how to build will be discussed in the papers included in this symposium."
J. Moir and J. A. Leslie, in a paper presented to the British Institution of Radio Engineers in London in 1951 stated:
"In a stereo system the acoustic characteristics of the studio can be faithfully reproduced and the long tunnel effect [of monaural systems] disappear. At first sight this and the concomitant advantage of being able to "place" the artiste or instrumentalist do not appear to be great advantages, but in practice they do produce a most remarkable improvement in the acoustic illusion.
A somewhat unsuspected result is the increase in clarity due to the spatial separation of the artistes, a result that is probably connected with the ear's ability to disregard sounds that approach the listener from directions other than than in which he desires to listen."
In 1941, Dr. Fletcher published a paper in the Proceedings of the Institute of Radio Engineers* which stated:
"This paper gives the requirements for ideal systems for the transmission of speech and music."
"During the past two years a survey of the hearing capability of persons in a typical population has been made by the Bell Telephone Laboratories. This was done in connection with the exhibits at the World's Fairs at San Francisco and New York City, sponsored by the Bell Telephone Companies. At these exhibits records of the hearing of more than one half million persons were analyzed.
Some interesting results from experiments in the reproduction of stereophonic depth and width were reported by Steinberg and Snow of Bell Telephone Laboratories. Using a subjective listening test methodology with 12 subjects, they found that a three speaker system provided superior depth and a two speaker system provided superior width.
The term "sound stage" was introduced into the technical literature in 1959 in a paper presented to the I.E.E.** Convention on Stereophonic Sound Recording, Reproduction and Broadcasting by British Broadcasting Corporation engineer T. Somerville:
"Sound Stage-It is proposed to use this term to describe the region between the loudspeakers in which the stereophonic images appear. The term "sound field" is deprecated because "field" has other connotations."
Two Bell Telephone Laboratories scientists, F. K. Harvey and M. R. Schroeder, presented a paper at the 12th Annual Convention the the Audio Engineering Society on October 11, 1960 with abstract as follows:
"In transmitting and reproducing two-channel stereophonic signals, the original program material may be modified deliberately or unintentionally. Separation upper and lower cutoff frequencies as well as full-band channel separation (in dB) have been evaluated subjectively in terms of detection of spatial difference, preservation of spatial resemblance, and listener preference. In addition, other pertinent observations on subjective aspects are reported."
In their discussion of their test procedure, Harvey and Schroeder stated:
"Critical listeners were sought in these tests because of a desire to set permanent standards. At the moment, only a small percentage of people fully appreciate high fidelity. Even less appreciate or understand stereo. However, there is a growing sophistication evidenced among users of stereo equipment. Anticipating the future, it seemed wise to avoid naive or unconcerned personnel in these tests to prevent establishing loose standards which eventually might have to be abandoned.
The listeners chosen were sophisticated in the art of sound localization either by working in this field or by education before testing. They were felt to be the equal of any serious listener who is accustomed to playing the same records many times and thus becomes familiar with the more subtle artistic and technical effects." 
Harwood B. Moore, a research scientist with General Electric, published a paper in 1960 which concluded:
"Subjective listening tests have been completed which indicate that stereo, in any of the forms compared, is preferred over monaural, but normal stereo from two full-range speakers well physically separated is, in all probability, the most preferred.
Listeners were selected from draftsmen, engineers, technicians, and secretaries. Only one listener was permitted in the testing area at a time. Each test required 15 decisions because it included six systems compared two at a time.
The listeners were educated to the extent that the only significant changes which should be judged should concern the reproduced sound panorama." 
*The A.I.E.E. and the I.R.E. merged in 1963 to become the Institute of Electrical and Electronics Engineers (I.E.E.E.), the world's largest technical society.
** British Institute of Electrical Engineers (not affiliated with the American I.E.E.E.)