The simple explanation basically aligns with
@Queen6. I did first look at the 507 and heard that's it's a noticeable improvement over the 300, for the most part since we're all different, and the consensus I got from digging into the 1A vs the 507 is that it is also noticeably better with someone saying the 507 is in between the two older models.
I think I heard that the 507 is quieter than the 1A in terms of noise but to me the 1A with my UE 18's is already quiet enough so I don't mind that.
I also got a 1tb memory card so I don't care for streaming and the battery life is astounding.
The bass is supposed to be better on the 507 but the 1A has better bass than than the 300 already so again, I'm happy with my choice.
I meant the 1A is way better than the ZX300 since I've never listened to the 507. I used the same DSD files across certain albums between the Walkmans and fresh out of the box the balanced end of the 1A spanked the 300. And now?
No contest.
To me its like going from 1080p with the 300 and going 4k with the 1A. I've personally never heard the amount of detail across almost all the music I listen to with the 1A. The 300 sounds kinda flat dimensionally and a little dense with instrument separation and this is something I noticed without any burn in on the 1A. Haven't done a comparison now at 125hrs.
I've also used it in my car on the auxiliary and the 3.5 also spanks the 300.
The 1A has finally started to show which of my flac files aren't quality. That's a first. Also, at least to me, as I burn it in more, I have to redo EQ settings and the adjustments become more subtle as I get closer to 200hrs. So that's proof to me that the sound is changing. The most I apply to any setting in the EQ is +3 with a few frequencies untouched.