Class A means that the same amplification device (in this case, tubes) are doing the entire waveform, both the maxima and minima of the wave. Class AB and Class B use separate amp devices to do the maxima (or top side) and minima (low side) of the wave; those are more efficient, but not nearly as accurate or true. Fully differential means that the signal remains balanced throughout, never becoming unbalanced or single-ended. Almost all amp circuits break the balance (in a console, the signal is single ended from the time it comes into the preamp, until bridged at the output.) This is accomplished by using identical, mirror image signal paths throughout – and why we use ceramic deck attenuators instead of potentiometers for gain adjustments. Common mode rejection ratios are significantly improved, as is signal to noise ratios.