For albums released in 1967 and especially in 1968, mono records are often worth more than their stereo counterparts. This is because mono records are usually mastered from tapes that have remained virtually intact for almost 50 years, resulting in editions that sound as good or even better than the original versions. In addition, many of these mono presses are much more affordable than their stereo counterparts, making them a great option for collectors. Mono discs are considered older than stereo discs, and having an older mono album is rare.
This is because artists and bands initially recorded older songs with mono audio, making them sound better in a mono format. Jazzbos tend to believe that the mono is better no matter what, and the original 1950s and early 60s editions of anything from Blue Note, Prestige, Riverside, Impulse and Columbia are more valuable. To my ears, mono records can sound louder, with instrumentation that sounds clearer and, at best, the sound may seem to be coming from the speakers. But how much better are they? I used to try to avoid stereo releases of jazz records from the mid-'60s and earlier because I was under the impression that they were a “fake stereo” created in dimly lit back rooms to get on the stereo bandwagon.
When stereo was still new, many record labels released “stereo demo discs” filled with vocals, orchestra sounds, and even street sounds to get buyers excited about the new format. This was problematic for buyers since few stereo discs had a prominent label in relation to the actual nature of the disc's content. As for qualitative hearing differences, the mono versions tend to sound more direct and have more strength since instruments tend to “compete for space and overlap”. On the other hand, stereo discs should not be played on a mono record player since a mono cartridge doesn't move both sideways and vertically, damaging the slots on a stereo LP.
Although these recordings weren't really stereo, they were packaged as stereo discs, often with a small note on the cover stating that the music had been artificially enhanced to simulate stereo sound. Since the stereo versions of many of these albums have been available and printed continuously for decades, stereo master tapes are often in poor condition leading to stereo discs being released today that don't sound as good as when they were first released. You can distinguish each instrument more clearly but they don't combine to make them sound as powerful as in mono. The audio reproduction of mono vinyl records revolves around sounds which produces a more impressive effect since all the musical parts compete for the same space. Record companies make money by selling both new releases and titles that are already available known as the “old catalog”.For record companies that meant buying expensive stereo recorders to record their music in stereo.
Of course panning instruments or adding stereo reverb will make the sound of a recording different from that of a mono version.