Phono Cartridge IMD Measurements, Observations and a Question

I've just recently added an IMD test to my cartridge alignment procedure. I use the REW RTA feature as a spectrum analyzer. It will display enough resolution to pick up the +/- 60Hz IMD sidebands at 4KHz using the AP Ultimate Analog Teat LP. (And it will go down to 2Hz, so the turntable rumble can be seen!). I convert the signal from A->D using a Benchmark ADC1.

I have an SL-1200G. I'm using two arms, a Hana ML on the stock arm and a Shelter 901 MKIII on an outboard pod mounted Micro Seiki MA-505 arm.

Signal comes from the cartridge to a switch that selects the arm, then into an SUT with Blue Cinemag treansformers., then on to a Parasound XRM phono pre operating in MM mode (40dB gain). Balanced cables come out and go to the input of a Levinson no. 38s line level preamp (oldie, but sounds and measures great). I then input the 38s signal to the A/D (balanced cable)

I measure about 0.5% IMD with the ML and about 3% with the Shelter. I have moved both arms up and down. Total vertical movement for each arm is 6mm. I cannot measure any change in IMD with either arm at any height when I change the height. Both arms have VTA on the fly, so I can observe the measurement as I adjust.

Why can't I see any changes in the IMD? Am I measuring the wrong thing? Should I be adjusting the rotational alignment of the stylus to the groove (offset angle)? Or some other parameter?