I felt that Sony not just perfected the DAP sound wise but also somewhat eliminated the need to seek the most high resolution audio files for very enjoyable highly music listening.
As I mentioned before, I find every audio file regardless of bitrate and sampling rate to sound really well rendered by the Sony Walkman sound processing.
With this firmware, I don’t find lossy audio files to sound that much worse than high res lossless audio files.
Yes there is still subtle graininess differences between true high resolution audio files and lossy audio files but what Sony has done is close the gap alot more that even 128kbps mp3 is also detailed enough to be very enjoyable to listen to.
I would describe the sound I am hearing from my IER-M9 as expansive surround, non-fatiguing, highly detailed with very accurate tonality and timbre. I would say that Sony has perfected the sound signature of the ZX507 to suit long listening sessions.
Also this firmware has excellent loudness control to a certain extent. Once you set a volume level suitable to your ears, you don’t have to bother about adjusting the volume anymore to compensate for loudness differences between different albums.