More likely the RCAs ground the negative phase of a balanced signal inside the chassis and convert to SE operation. That’s what Atma-sphere does with their amplifiers, so those who have SE components upstream can stay SE in the amplifier.
@lewm Either I'm misunderstanding this or its incorrect. At no point in our amps does the signal go single-ended. What we do instead with the RCA connection is the center pin connects to pin 2 of the XLR and the RCA ground is pin 1 of the XLR. We then ground pin 3 of the XLR if the RCA is being used instead of the XLR.
A differential amplifier, present at the input of all our circuits, then converts a single-ended signal from the RCA to balanced operation. No transformer needed. The differential amplifier doesn't care much if the input is single-ended or balanced, since it simply amplifies what is different between its two inputs. If one side is ground, so be it.
Also, on the page with specs, the gain is not different when you feed the RCAs vs the XLRs.
If the circuit and source are true balanced, and if one side of the source is grounded or not will make no difference to the gain if the preamplifier is really balanced. The reason is the same amount of Voltage is applied to the input in either case. So if a phono cartridge has 1mV output, if that 1mV is applied to a balanced circuit via an RCA connectors (so the inverting input of the balanced circuit is not used) the gain will be the same unless the balanced circuit is not differential or having a transformer coupled input. Since the CMRR our our phono section inputs is rather high the signal level at the output of the preamp will be unchanged whether the input is balanced or not.
There is a common misconception that a balanced signal will be 6dB more gain but if the source is AES48 compliant that simply is false.