Does the dip at 125hz go away if you pull the speakers out further?
I did not try that with the LFR again in part because there is a limited distance from that wall to the seated position and reducing that distance makes the stereo imaging worse, BUT, it is notable that the M60 V4 has a hump at that same frequency and is the same distance from the back wall to the baffle. Same goes for the M60 Ti (has a significant valley around 95 Hz, not 125). If this was specific to the LFR880 then, well, we're all at a loss to adjust specific speaker distance for an exact room. Move a few inches in one direction, you get a valley at X frequency, a few inches another direction, you get a valley at Y frequency.
Just guessing, but SBIR at 125hz is roughly 26” . No corners to obscure comb filter in this setup. I see it repeating at 125hz intervals. Curious. Is your baffle 26” off front wall?
It is close to that for the M60 (about 26-27" with the toe in) but the 880s were out by another inch because of their size (was trying to balance front baffle without them being too close to the back wall as they are deeper than the M60s). As mentioned though, if the 880s suffer this, why did it not show at that frequency for the other two M60s?
Seems odd no?
