You might also try deliberately twisting either cartridge a bit so that azimuth is out of alignment. If you still see no change then you know there is something amiss with your test setup.
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?