Typical audiophile: nice gear in a poor (smallish/square) room.
Who told you that you needed a sub (let alone a pair of subs) in that room with the ATC SCM40?
Recommend trying a non-symmetrical layout, centered very roughly in the left corner and adding GIK 244 absorption panels on two adjoining walls to break up the echo (that would be driving me mad).
Read Floyd Toole’s “Sound Reproduction”. In it he describes bass behavior in a room as behaving like waves in a shallow bathtub. As you move your hand lengthwise the waves hit the end and bounce back until they encounter another wave, where they cancel, add, or just interfere. So you want the bass generators (speakers/subs) to NOT be side by side, and instead to be dispersed even flipping phase on one of the subs.
So recommend moving the left sub farther back in the room, locate by putting that sub (and disconnect the other while doing this) in your listening position, then crawl around the room to find where it sounds the loudest/clearest - that’s where it should go.