What should I be looking for in REW?

[Snoochers]

Your original graphs, to me, look terrible between 3k and 20k...And lack of bass below 150hz would concern me.
You also have a lot of pretty wide q (width) of bumps and valleys. the wider the bump / valley is, the more you will notice it.

Thanks for this. I will do some measurements again with just one channel going at a time to assess the situation more accurately and report back.