It seems that iBasso have turned off attenuation of the DAC signal with 1.2.3beta. That is technically a good thing to do and it explains the increased dynamic range.
Query: have they otherwise increased treble directly, activated DAC pre-ringing, or done some other further modification to boost treble performance? I have no idea. The increased treble snap is driving increased perception of soundstage width and overall space. I am not entirely in love with that particular change. If they have done something to enhance the treble other than not attenuating the DAC, I hope that they will roll that back as it's really the audio equivalent of a parlor trick rather than an actual increase in sound quality.
The player performance is otherwise greatly improved. The DX50 is actually easy to turn on now! Media scanning when I browse is almost instantaneous. Bravo on both counts. I would hate therefore to go back to 1.2.2 just on that basis.
I am certainly going to stick with 1.2.3beta for now so I can see what further refinements may be instituted with the official release on a direct comparison.