It's hard not to be impressed by a DAP that works perfectly out of the box. My DX50 came with 1.2.2 already installed. I plugged in my 128 GB FAT32-formatted HDXC via the Sintech adapter and the DX50 asked if I want to scan or browse by folder. My files are all sorted by Genre/Artist/Album so I chose browse and it works... well... perfectly. I'm a Rockbox fanatic but really this GUI is pretty sweet - if a little more fidgety with clumsy fingers.
Regarding locking but leaving the volume control available, if I touch the power button to turn off the screen, that is what I get. Yes the three big buttons still work but that doesn't seem to be a problem.
Initial thoughts regarding gain: it actually seems to affect power output. So on a sensitive IEM like the Heaven VI, I get a thicker bassier sound on high gain but more delicacy and air on low. The high gain is a bit more of a forward sound - excellent with the KAEDE.
I have tried amping the line out a bit but I am thinking that the DX50 has a pretty good amp already. There is plenty of power. It is way to early to reach any final conclusions though; I am just a getting started.
Overall I am very pleased. It sounds great with both the Heaven VI and Flat4 KAEDE.