How do I fix phase cancellation on wide synths in mono?

To fix phase cancellation on wide synths in mono, use a high-pass filter on the side channel of a mid/side EQ to cut low frequencies below 150Hz, which keeps bass frequencies centered and prevents them from canceling when the track plays in mono. I've found that combining this with a shelving EQ approach works even better: pull down the side channel by about 40dB with a shelf around 400Hz, then boost the mid channel by the same amount to preserve the unique textural qualities of the synth while maintaining mono compatibility. You can also push the mid channel up about 3dB and pull the sides down 3dB using a mid/side balance tool to make everything fold almost perfectly in mono while keeping your stereo width.