Eldartford,
If you are measuring the output of your phono stage, you better make sure that the phono stage bandwidth is not limiting (mine is rated flat out to 100 kHz). Also, if you are using a moving magnet cartridge other than a Grado, the high inductance of its coils will roll off the treble rapidly, sometimes starting below 20 kHz. This rolloff gets worse with increased load capacitance.
If you are measuring the output of your phono stage, you better make sure that the phono stage bandwidth is not limiting (mine is rated flat out to 100 kHz). Also, if you are using a moving magnet cartridge other than a Grado, the high inductance of its coils will roll off the treble rapidly, sometimes starting below 20 kHz. This rolloff gets worse with increased load capacitance.