Acoustic Timing Reference for Subwoofer Measurements

You can make the center channel the reference. Or left front. Or the right front. To measure only the sub without the front channel, physically disconnect the front channel from the signal, leaving the sub.

When you want to measure the left front sub, physically disconnect the left main, but leave the left sub connected. The reference will be the right main one. With the reference right main this way you can measure the left front sub and the two rear ones. Then switch roles. This will measure the right front and two rear ones. You can also physically disable the front subs to measure the time between the front mains. After measurements, you will need to work with the overlay window to understand how all six speakers are coordinated with each other. It's complicated. To simplify, you need a separate, seventh speaker with a tweeter, which you will make the reference.

The second half of this video demonstartes using REW Electronic Delay Measurments in a 2.2 stereo configuration