Vortex Sound Research

I'm fairly new to analog synths. I got a Neutron less than a year ago, and recently got a Behringer Model D. I have played the Model D a few times. I tune the oscillators each time I play it after the synth warms up. I have not calibrated the synth yet. Whenever I turn the Mod Depth knob up just slightly greater than zero, the sound goes way out of tune, regardless of what other settings I have setup, such as whether OSC3 or Filter EG is selected, or whether Noise or LFO is selected. Is this expected? Is there something I can do to make it more subtle?