Frequency response off with test record

Nothing to do with loading which affects treble region. Also given your vague description you might be looking at a benign bass rolloff. At what exact frequency? Is your scope calibrated? Are you accounting for the RIAA effect on frequencies below 500Hz?

Depending on your scale this can look pretty scary but you may only be down 3 to 6 dB at both ends which would not be out of the realm of normal.

I wasn’t thinking about it being a logarithmic scale, so it’s not as dramatic as I thought.. The change was close to double in amplitude, so approaching 6 dB. But, I thought typical frequency responses were flat within about 1 dB for cartridges.

At what exact frequency? Is your scope calibrated? Are you accounting for the RIAA effect on frequencies below 500Hz?

Unfortunately, I haven’t had time to go and measure the exact frequencies at which this occurred. That would require me to actually do some work :), as they don’t state the frequencies on the record. It is however midrange/upper base, and not low bass frequencies that are elevated. I’m going from memory of the waveforms on the scope that it was probably around 250-500 Hz where it was elevated. It does roll off after the low bass as well.

I am measuring after the phono stage, in the tape outputs, so shouldn’t the RIAA effects already have been compensated for?

"around" is not good enough. Get a test record that reports the frequency such as the Hi Fi News Analog Test Record so we have a better idea what is going on. Right now just listen to music and enjoy. In the end what are you going to do about this, get a new cartridge? I think not, especially if you like the sound of the one you have now. 

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.

The test record I purchased had a spindle hole so off center the tonearm was wagging in and out about 1/8" every revolution.  Speed, wow and flutter tests looked incredible. LSD incredible :)