i agree wholeheartedly that the human ear is an incredible piece of acoustic equipment. when combined with the human brain, its probably one of the greatest acoustic processor designs on the planet!
in the context of this conversation though, the system is badly biased by the fact that the sound generating apparatus (harmonica, jaw, throat, chest, skull, sinus passages, etc) are mechanically very closely coupled to the acoustic system, in particular the microphone (ear) that the acoustic processor is no longer able to accurately judge the sounds its being presented with.
its somewhat akin to gluing microphones to our skulls to pick up the trumpet we are playing. we'll get a signal, but certainly not the signal coming out of the bell of the trumpet.
the tape recorder allows our acoustic processing system (ears and brain) to be able to perceive sound much more objectively than listening to one's self during the act of making sounds, either instrumentally or vocally.
additional to this is the effect of psychoacoustics, the effects on the brain's acoustic processing abilities effected by inputs and environment not necessarily of acoustic origin. for example, the ability to blank out certain sounds as being unimportant due to constancy of the sound and the lack of impact on the human system. or brain workloading, where the brain simply ignores inputs once a threshold of the number and speed of inputs is exceeded.
i guess my point is, as a musician, i'll be able to judge my tone quality, and the quality of technique much better (more truthfully) from a tape recorder than by what i hear while i'm playing.
as an afterthought: the acoustic reception and processing capabilities of today's sonar systems found on american submarines far exceed the capability of the human ear in both sensitivity, accuracy, and processing. just sayin.....