Since REW is a mono signal, you can't utilize any soundfields in your processor. You must use stereo mode only. This means the two mains and sub will be active, and none of the other speakers will be active.
To measure a sub, disconnect the mains speakers. To measure the mains, turn off the sub.
If you wish to measure the rear speakers for example, connect them in place of the mains.
If you wish to measure the center speaker, connect it to the right or left main position and shut the other mains off.
When you measure the mains, use the usual vertical scale of 45dB-105dB, and use a horizontal scale of 15Hz-20000KHz (if you have a full range mic).
Yes, for full range plots, you need to turn on smoothing to hide the comb filtering (scratchy).......