The actual circuit used is single-ended Class A.
What each signature refers to is a harmonic envelope and relative levels of harmonics. These are achieved by adjusting the open-loop gain and thus the amount of negative feedback in the circuit.
So the difference between ‘classic studio’ and ‘Push-Pull’ is in the levels of harmonics, which are greater for ‘Push-Pull’ (just as they would be in a Push-Pull tube amplifier vs. a studio pre-amplifier) and feature approx. equal amounts of 2nd Order Harmonics and 3rd Order Harmonics and similar relations between the even/odd harmonics at higher harmonics.
For the ‘single-ended’ signature overall 2nd Order Harmonics is much higher than 3rd Order Harmonics and other harmonics fall off in a similar manner with the even-order harmonics in each pair significantly higher, as would be the case with a low/non-feedback SE amplifier.
