I could not find anywhere the rca preout voltage for nad c372. Could 7v you mentioned be the output voltage of another model through a xlr preout perhaps? On the other hand from the specs of some other nad amps indicating 1.2v rca pre output, I thought that the rca pre output of c372 might be 1.2 volt.
I really don't know how many times I have to repeat this (sorry don't mean to be rude, just frustrated), that one cannot know what exactly those pre-out voltage specs provided in the manufacturers data sheets, manuals etc actually mean, unless they also provide other pertinent info such as the test load impedance, THD+N, frequency range, continuous, average, or peak values, what (if any) test protocol and/or standards they followed in obtaining the figures.
It is entirely possible, for example, that the Yamaha RX-A30X0's rated pre-out voltage of 1V (under conditions blablabla), if tested by Gene using his AH test protocol in his lab, could actually output higher unclip voltage than another brand's AVR that specified a much higher voltage but under different conditions or stating nothing but just something like preout voltage: 7V, without telling you anything else.
Even if one wants to compare maximum preout voltages using actual test results, one will have to stick to one particular lab, preferably by the same person. That would include measurements done by Gene of Audiohoics, and JA of Stereophile and to certain extent, and AVTech/Miller audio research.
Now back to the NAC C372, if you can trust Steophile's, below is the link to JA's measurements.
NAD C372 preamp voltage gain
Based on that review, the C372 has a preamp voltage gain of 9.05 dB, that means if you input 1V to the preamp section, you get about 2.83V. Now that does not mean the NAD can output 2.83V unclipped, we do not know that because JA did not say anything about testing the preamp section until it clipped.