The 417 / 5842 pre is plate transformer coupled , there is also the choice of balanced ( low Z input) or unbalanced input ( high Z ) . The measurements were done using the balanced ( transformer coupled ) input and the balanced output @ a level of 5 Vrms.
Although there is no frequency correction provision in the pre , note that the frequency response is flat from 30Hz to 30Khz and only @ 17.6 Hz it is down 1.9 db.
Here is the THD @ 5 Vrms output.
This is a picture under the hood just to see that no fancy transformers or exotic parts were used.