After some studying, this is what I found:
For the Allegri Reference passive, autotransformer volume control preamp:
Input: pseudo-balanced input, with on pin 2 of the XLR signal, and on pin 3 ground. [Pin 1 (ground) on the balanced XLR out from the dCS source is not connected, so ignored] Maximum signal level: 4V RMS 8Hz; 10V RMS 20Hz and up. At -10dB (normal listening) input impedance is 200kohm, for a power amplifier load with 20kohm input impedance.
Usual balanced pin assignment: Pin 1 ground - Pin 2 positive signal - Pin 3 negative signal, so the Allegri drains the negative signal from the dCS to ground. A simple method of driving a balanced line is to inject the signal into the “hot” wire through a known source impedance, and connect the “cold” wire to the signal’s local ground reference through an identical impedance. Due to common misconceptions about differential signalling, this is often referred to as a quasi-balanced, pseudo-balanced or impedance-balanced output, though it is, in fact, fully balanced and will reject common-mode interference. Internally, the Allegri’s design remains single-ended throughout, since there is no negative signal coming in, as opposed to a differential amplifier.
Output: transformer based floating balanced XLR pair, and unbalanced RCAs. This practice also assumes that the balanced input of a following power amplifier has proper differential summing.
To maintain the integrity of the Fractal™ wired transformers, Townshend strongly advises to use their Fractal™ Interconnects for both the input and output cabling. The balanced version of Fractal-Wire™ incorporates two strands of the ultra-fine enamelled copper conductor spiralled together with a parallel drain wire but no shielding, it is terminated with Neutrik XLR plugs. In the unbalanced version, the signal travels through a very thin polyester coated enamelled copper wire, which is spiralled around a much thicker bare copper ground. But technically, all standard XLR cables (balanced pin assignment: Pin 1 ground - Pin 2 positive signal - Pin 3 negative signal) will work.
The Allegri has no active or amplifying audio electronics in the signal path. However, the sources used must have sufficient output to drive the following power amplifier via the controller to the required maximum output power, with some headroom for those quieter recordings.
So, if I understand correctly, setting up the Allegri between a dCS source and a power amp, you will get best SQ when using XLR cables for both its balanced input and output. 6V out from the dCS source will work best. Depending on the balanced input sensitivity of your power amp, you can set the Allegri’s minimum attenuation according to the table posted above, using the formula also posted above to determine its line out voltage.