That is the best room shot but with the older setup of home built, JBL plans speakers on the very right side next to each other. They were my first real L/R speakers a long time ago. Walnut, solid core ply, etc. They are replaced with one JBL and collecting dust in the garage even though they are covered.
You can barely see one of my passive subs in the far left side under the wall clock. the other is better seen in one of my sig line file pictures. 15" JBL, the other is an exact copy. Now they both have upgraded drivers.
No I don't hear the piano feedback when the subs are going. Probably masked.
1 1/2" concrete under carpet and padding, just 1/2" but thick and dense carpet.
Plenty of room of a grand piano though. One in the picture is a baby grand, wife's family, grandparents I think. She plays it now and then.