To notice this preference you must have a really good headphones/iems.
I can agree that the amp12 has bit more detail/etch. Detailing or low level resolve is not lost on the amp13, just different smoother presentation.
After listening to:
Amp11 (ok)
Amp11mk2 (better)
AMP11mk2s (good)
Amp11 whitigir mod (top notch!)
Amp12 (good-)
Amp12 EXN (good+)
Amp12 EXN-S (top notch!)
Amp13 (good+)
I prefer without hesitation the modded boards.
Whitigir coming out with even newer "muse" moded now, & also both modders are doing the amp12 ex -"Silver" versions. It doesn't stop with stock.
To me, both the AMP11mk2s and amp13 (not pictured) are also great and above average, but not best you can get.
Using my Mest Indigo IEM was most revealing trumpet/strings edge detailing.
Users with smoother IEMs may not notice as much. Smoothness does not mean less resolve, so I'm not saying that. Some IEMs have more value in presentation and musicality.
I still own both dx300 & dx320 to compare:
I find no matter the setup of players or amps, the dx300 retains a bit more tonal thickness to notes, while the dx320 is less colored DAP, while retaining naturalness, musicality, and a touch more low level dynamics & depth.
The amp boards and music players make the dx300 to catch up/ get closer to the dx320 in those two latter aspects.