It means the NAD has a very wide dynamic range. Don't worry about it. Set the volume to the level you like and don't worry about what the numbers read...
It is not referring to sound pressure levels (SPL), which are commonly expressed in db as you realize. I suspect it is referring to the relation between the signal voltage at the output of the preamp section of the receiver, and the signal voltage that is being sent into the receiver.
db's are always derived from the ratio of two quantities. For voltages, the number of db is equal to 20 times the logarithm of the ratio of the two voltages (the number 20 becomes 10 if power levels, rather than voltages, are being compared). As a consequence of that, a setting of 0 db would mean that the output voltage of the preamp section is equal to the input voltage; +17 db, representing 17 db of gain, would mean that the output voltage is about 7 times as great as the input voltage; and of course -90 db, representing 90 db of attenuation, would reduce the signal voltage and hence the volume to essentially nothing.
SPL db usually represent sound pressure levels relative to a level of 20 "microPascals" according to this Wikipedia article, approximately corresponding to the threshold of human hearing.