In fact, one of the greatest difficulties concerns the mentally separating underlying quality characteristics from simple first-order errors such as frequency-response aberrations, loudness and channel imbalances.
Slight errors in channel balance, either in specific frequency ranges or in overall level, can subtly disturb one's opinion of the sharpness of stereo focus. Statistically well-controlled testing has not only confirmed the audibility of absolute phase or signal polarity but also level differences as small as 0.2dB. These differences may range over octave or several-octaves of bandwidth, with an aural sensitivity of a similar magnitude. The subjective responses to variations in amplitude/frequency response are pretty well documented; the careful reviewer bears these constantly in mind. For example, less than 0.5dB, just 5%, of treble lift in the 3-10kHz range can give rise to a mildly increased sense of immediacy, transparency, and liveliness without necessarily being directly obvious as treble lift. A similar degree of loss in the 150Hz-400Hz range can make a vocalist appear lightweight and lacking in power in their fundamental range, conversely lending a crisper quality to the sound. This might be preferred on one recording but disliked on another.
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Greatest awareness of the long-term quality of an item is generally obtained by the single presentation method, while maintaining critical control of absolute level and channel balance, combined with an awareness of any relevant response errors.
