One more comment. Since the discussion started with standing waves; the front to back standing waves in this room will be at multiples of 67 Hz. The presence of the reflection induced dip is tuning the first mode a bit, so L and R look different. The two peaks at about 134 Hz and 201 Hz in both the L and R response curves are a result of this mode. Mounting panels on the wall behind the speakers should help tame these peaks - see the photo in the post I linked in another comment for how I have my room set up. (The black panels behind the speakers are GIK monster traps. The big panel on the left wall is an Acoustimac 2" art panel.) It will also help with decay times, which will improve the listening experience.

Generally speaking, in a room this small I would focus on treating first reflection points with the panels, rather than going after corner placed traps. The wall behind the speakers is both a first reflection point and a big contributor to long decay times.

> Do you mean that those dips may decrease if I measure both speakers at once? (It’s not that bad as it seems?)

Yes, for bass frequencies for most music the mix is mono, so an L+R measurement will be more representative of the listening experience. Since your dips are at two different frequencies the impact could be quite significant - lazy style, if the sound from one speaker is cancelled perfectly at the dip (say a 20dB deep notch), with a second speaker at a different location providing half the total signal power will mean that the total SPL at that frequency only dips 3dB.

I suspect that those narrow dips are due to first reflections off the side wall opposite each speaker (or perhaps the ceiling). So for the right speaker, there is a primary reflection off the left side wall that arrives at the MLP with a 5msec delay, which is a half wavelength at 100Hz, so the reflected wave cancels the primary wave. I had an issue like this in my listening space that was due to a ceiling reflection. If this is the case, you will get a better improvement by placing the traps at the first reflection point. As an example, I shared pictures of my installation here.