In 2010 I contacted Ron Streicher, co-author of The New Stereo Soundbook, as well as President of the Audio Engineering Society in 2003 and 2004. I had sent Ron some of my stereo audio samples, upmixed from mono sources through the use of spectral editing, which I had posted in a thread in the "Unconventional Stereo & Pseudostereo" Forum that I moderate at the Both Sides Now Stereo Chat Board (free registration needed to view and/or post to the chat board). Ron was kind enough to give them a listen. Although Ron didn't wish to participate in the ongoing discussion thread at that time, he had asked that I post the following there on his behalf:
"The fundamental difference between mono and stereo is one of spatial perspective: because it employs only a single signal channel for recording and reproduction, mono can convey an audible image reliably in only one dimension - that of front-to-back depth. Since by definition stereo employs two (or more) signal channels, it can add the dimensions of width and even height. These are the basic technical considerations. How they are implemented, however, is what turns the technology into art.
During the seven decades since the pioneering work of Alan Blumlein and Harvey Fletcher, stereo technology has expanded from basic two-or-three microphone (and loudspeaker) perspectives to multi-channel systems involving even dozens of microphones and loudspeakers. Given the evolution of different philosophies regarding what constitutes "true stereo" it is no longer possible to place arbitrary limitations on what techniques legitimately can be utilized to achieve any audible recording or reproduction experience. In this context, "anything goes" as long as it produces the desired results. (Note that the critical word here is "desired.")
When it comes to deciding what is desired - or desirable - this also must be placed into the proper context. Contemporary commercial recording studio practices have evolved since the days of Les Paul to enable the recording producer to place sound elements anywhere in the soundfield to create a sonic experience that either could have happened in a "live" performance (a "re-creative" approach) or that does not happen until it emerges from the loudspeakers (a "creative" approach). Both are equally valid in their own right. Again, that is where technology merges with art -- and art is in the eye and ear of the beholder.
When converting "historical" monophonic recordings into stereo, artistic integrity to the original source requires that the musical intent not be unfairly distorted: the ensemble, style, and performance that were captured originally must be preserved in a "realistic" manner so that the music remains the focus of the listener's attention -- not the techniques used to reproduce it. (This is similar to the "fourth wall" in the theatre: if the audience becomes aware of the stagecraft, the content of the play is lost.)
Given the advances in digital technology we soon may reach a time when the conversion of historical mono recordings into "seemingly-true" stereo (or even full multi-channel surround) is as commonplace and effective as the multi-track-mono recordings made daily in recording studios around the world. The recent experiments by Christopher Kissel is a clear indication that this is the direction we are heading."
Ron's eloquently stated considerations for those upmixing older mono content to stereo should serve as excellent guidelines for insuring that the resultant stereo mixes remain true to the spirit of the original mono source material. I appreciate that Ron took time out of his busy schedule to pass along these words of wisdom!