In a balanced preamp the phase switch simply redirects the inverted phase to the non-inverted circuitry and vice versa. Its completely passive.
Nothing is completely passive. The more revealing the circuitry, the more it exposes component 'flaws'.
Experience with phase flip switches in active and passive balanced hardware often left something to be desired in terms of routing and hence inaudibility.
Making both halves of a balanced circuit identical is EXPENSIVE. Add in tube vagaries and the probability of a sonic delta is fairly certain. Not to all listeners, but to some.
Decades back it was demonstrated there were distinct sounds in a system on the same source:
- Normal polarity to speakers
- Invert channel A at source* and speakers - same Φ as 1
- Invert channel B at source* and speakers - same Φ as 1
- Invert both A&B at source* and speakers - same Φ as 1
Theoretically all should sound the same, but they didn't. If one subscribed to OOPS, at least 2 & 3 should sound the same, but they didn't. It was postulated that the loudspeaker is an asymmetric load and depending on the other hardware, affects the sound.