Through my Pass XP10 inputs 1 and 2 (both balanced inputs) when you switch back and forth, the Parasound is about 6 clicks louder on the volume knob (6 dB?).@marlin38 Yes, according to the specs and the manual for your preamp 6 clicks corresponds to about 6 db.
My suspicion is that the reason for your findings is that one of the two signals in the balanced signal pair that is being provided by the Theta is not working (i.e., is zero volts). For a given setting of the volume control on the preamp that would result in 6 db less volume than if the Theta was working normally.
If you have a multimeter you could disconnect the Theta from the preamp, play some music with it, and using an AC voltage scale on the meter that is suitable for measuring a few volts measure the voltage between pins 1 and 3 of the Theta’s XLR output connector, and also between pins 1 of 2 of that connector. If my suspicion is correct you’ll most likely see (depending on the internal grounding configuration of the Theta) that the meter indicates a significant voltage for one of those measurements (fluctuating with the music), but zero (or close to it) for the other measurement.
A magnifying glass may be helpful in reading the pin numbers that are marked on the connector.
Regards,
-- Al