This is one of the reasons Copland does not want the unit connected between the amplifier and preamplifier. It is equivalent to a line level device (such as a CD player) and does not have the voltage swing of an active preamplifier.
The input/output voltage of 1 volt nominal indicates unity gain. Input/output impedance is not given, although it would not be difficult to determine. Typical values are 47 K ohm or more at the input and 1 K ohm or less at the output. My manual states maximum output as being 2.4 volts.
Unless your passive preamplifier uses an autoformer, it does not have voltage gain, meaning that whatever voltage is produced by your line source must be sufficient to drive the power amplifier. Typical line level sources, such as CD players, have outputs that fall between 1 and 2 volts.
Is it possible that your power amplifier is unusually sensitive? If so, you might be able to insert the DRC between your passive preamplifier and amplifier (assuming suitable impedances). Hope this helps!