The output of the phono stage should be flat.
To do this its electrical response should follow the RIAA curve.
(edit : The RIAA curve accounts for the fact that MM and MC output
is determined by tip velocity, unlike ceramics where output is due to
tip displacement. The width of the groove at various frequencies is
kept ~ constant ( for a given level ) consequently the output of an
MM or MC rises considerably across the frequency range, the RIAA
curve in a phono stage is the inverse of this output characteristic.)
The pre-emphasis and pre-distortion put onto certain records increases
as the tracked radius decreases, to follow the distortion and tracing loss
characterictics of a typical 0.7 thou spherical stylus.
Certain records only used pre-emphasis, zero at the run in
groove and 2 to 3 dB treble boost by the run out groove.
There is nothing the phono amplifier can do to correct this. To correct
it it would have to be done in the turntable and involve a tranducer
that knows the position of the tonearm, a crude method is a variable
resistor connected to the vertical axis pivot.
So the phono stage has to follow the RIAA curve, which is not flat.
Consequently phono stage response is given in terms of deviation.
With a perfect test record and cartridge the phono stage output is flat.