I think you have a point: If you are going thru the phono section, then RIAA correction should have been performed on the signal from the LP. So, next question: Does the test LP claim to be useful for your purpose? It would be tricky to create such a test LP, in part because of the RIAA correction below 500Hz. (The RIAA filter in your phono section boosts bass below 500Hz, is flat between 500Hz and 2kHz, and then attenuates the treble above 2kHz.) This is because the voltage output of an MM, MI, or MC cartridge is proportional to stylus velocity. Stylus velocity is proportional to frequency. If you were to feed the phono inputs from an audio signal generator, which is linear, you will graphically see the effects of RIAA correction on your scope.
Frequency response off with test record
I was running some tests with the Analogue Productions Ultimate Test record, and on Track 7, side 1, I found that my frequency response seemed off, peaked somewhere in the middle of the 1 kHz to 20 Hz range, in about the 3rd to 6th frequency down. Is this likely a cartridge issue or a preamp/loading issue? I was measuring with an oscilloscope from the tape outputs of my preamp. The cartridge is a Transfiguration Phoenix and the preamp a Spectral DMC-20. If I remember correctly, I think the cartridge load is about 100 Ohms.