I replaced my between speakers GIK panel with a GIK Art Panel. Used a picture of our cat. Wife loves it!
GIK has great products and are reasonably priced. I love the Art Panel!
I have four GIK Tri Traps and four 244 panels not pictured in Guilford of Maine Silver Papier. The room is open to the main level on right side which adds over 10,000 cubic feet so the subs help shore up the 25-35Hz range (I like some pipe organ music now and then). 14' ceilings throughout.
I had treatement behind the Salk SS8's but prefer none. I use the open back behind the mid-range option with some polyfill.