I blame
@Squishman and ALL CH STEREO mode for this drama.
The f#%kin’ DSU is what it is. Different chips do not mean the DSU works differently in one brand AVP/AVR from another brand AVP/AVR. No up mixer from any AVP/AVR can work a miracle on a sh#%ty old mono track slapped on a DVD that happens to output it as Dolby 2.0. This is assuming the player is outputting the bitstream and not converting the signal to PCM for output. That can actually be an improvement to output when playing sh#%ty non atmos signals from some discs.
Now, there are cross up mixing and speaker output characteristics that do differ from different AVP/AVR brands. If signal output, sound mode and speaker configurations are not set correctly for optimal results, it can be a mess. Speakers should be placed as close to optimal configuration layout as is possible to do so. The amp assignments should matched the connected speaker terminals. Crossovers should be set appropriately for a given speaker and that setting may not be the setting spit out by room correction.
After having done all that can be done, just don’t expect a f#%kin’ miracle and don’t use ALL CH STEREO mode for anything other than two channel music while eating chips and dip with friends or tinkering on something. Then, change sound mode accordingly for a movie or TV show if you want dialogue just from the center channel speaker.
Things can get heated around here when somebody over indulges in the sauce from time to time and then interacts with somebody who is self important ALL of the time.