You need more room damping, you are getting a midbass bump which will likely be with most speakers putting out anything significant below 100 cycles. I have heard the MBL 111, and they are very nice, but sound flat if only playing at a modest level. They sound best with their ears folded back and cranking it out, then the stage is glorious, but lacking the delicacy of your 20T which is a speaker I like very much. (I had the 10T, another great one, and it was short a ribbon tweeter from superb.) But, as the level goes up, so does your bump.....I would work more on your room, and I even apply under tables, and counters with Auralex type of foam, and this sucks more out of that standing wave. A small room is easily overloaded in that region. I even line the top of cabinets that do not go to the ceiling and this is invisible and takes up no room.
Jallen
Jallen