[brucek]

Is there a way to mute the left and right channel in REW?

Use the Windows mixer balance control. Or just connect REW soundcard to left and right channel separately.

Have a huge dip at 150Hz. What could be causing that?

Cancellation from a reflection or from the other speaker. If the direct signal and a reflected signal arrive at 180 degrees out of phase, they will cancel.

brucek