Seems my steppers just don't like spreadcycle. Changing vref did little to help. Since I installed all the cfg pins on the TMC2100s, without having looked at the code in AstroEQ or the datasheet for the TMC2100s, I just pulled the jumper pins to force the tracking into stealthchop. Slewing is still a bit whiny but tracking is now dead silent. At some point, I will probably just remove both cfg pins and see how the system works completely in stealthchop (plenty of torque in the custom gearing, I think). We'll see how that works since I can't program to save my life!