Nutra, acoustic treatment for listening rooms is very different to recording rooms. When you set about making a small room neutral for mixing, each new installation of traps, reveals another problem, previously unheard.
The final result can be very dead. Fine for mixing, but useless for drums.
Try some large sheets of plywood, laid at an angle to the wall. This should liven things up a bit. The angle will prevent flutter.
Drums in the corner, facing out, I would guess. Bass boost from the corner, like a Bass Horn. Plus all the reflecting walls are all farthest away from this position.