A push-pull amp topology requires two signals 180 deg out of phase. Each phase feeds either the push or the pull section of the output stage. When driven by a single ended signal, the push-pull stage is always preceded by a phase splitter to generate the correct signals. A balanced input can bypass the phase splitter since it conveniently contains two signals 180 deg out of phase. This saves a bit of active circuitry.
For single ended amps like the Alephs, the situation is reversed. A single ended signal goes straight to the first gain stage, while a balanced signal must go through additional circuitry to subtract the negative phase of the balanced signal from the positive phase, thus achieving common mode rejection and a doubling of the signal voltage.
Pass claims to do this via a passive network. I can't think of a passive method other than a transformer.
A single ended connection lets you avoid this passive circuitry at the expense of common mode rejection. For short cable lengths I don't think noise pickup is much of an issue and I'd rather avoid the extra circuitry.
My system plays both sides of this equation, A Pass XONO crossover high pass section feeds the Aleph 1.2 monos via a single ended connection, which in turn drive the ribbon panels of a pair of Apogee Studio Grands. The low pass section of the XONO drives a pair of Aragon Palladium II monos via balanced connection, which in turn drive the integral Studio Grand subs. The Paladiums are a tradition push pull design that omits the phase splitter and requires a balanced signal to operate (the single ended models just add a phase splitter).
One last thing - the first generation Alephs used a separate pull stage that took effect when the amp approached clipping. These operated push-pull over part of their range. The second generation (including Aleph 1.2 and 5) dropped the pull stage and simplified to 2 gain stages. They never move into push pull operation under any conditions.
