Other than tuning or changing the kernel for better battery life, optimization etc, is there anything we can done on the sound quality / signature, something like what the Fidelizer ROM is doing?
Yes, and there are two methods, specifically which I wanted to discuss in this forum among true audiophiles:
1) Third-party plugins (a la Viper4Android)
These are user-(or system-)libraries that are inserted via an apk or installation method that, depending on how the ROM is configured, can alter system-wide sound. Originally I was trying to get these to work with the FiiO, though subsequently I've learnt about the audio distortion they introduce, particularly because of downsampling and/or bandwidth reduction to process streams in as much real-time as possible. In theory these can still work, but the base ROM (not the kernel) needs to be configured to allow third-party software to mess about with audio_effects and audio_policy without being blocked. Previous versions of FiiO's X5iii (and X7ii) ROMs prevented this, but now that the V4A partnership has ended, it should be possible for these third parties to work provided the restriction has been lifted. I asked
@FiiO about this yesterday but haven't yet had any response. If anyone on 1.2.2 has managed to do so, please let me know.
2) System library modifications (a la soundfx)
This is quite a controversial (ie: love it or hate it) process, but has been successfully demonstrated on other forums. I have the files required to do this, but have refrained from doing so because (i) I've been busy on making the player faster and better, (ii) there's a purist angle here that I'm not entirely familiar with. All Android-based players have a sound-envelope that can be (and usually is) modified by specialist hardware vendors, such as FiiO, IBasso, Cayin, A&K etc. In theory it's perfectly possible to substitute these files from the different players and see how they sound, but crucially the sound envelope should normally be tuned to match the characteristics of the underlying hardware. Also you'd need to backup the original sound libraries (or reinstall vanilla) if you're not happy with the 'imported' sound.
So, I can do 1) depending on how much FiiO have changed in 1.2.2. And 2) easily, but I'm reticent to do so because - well, it's pretty clear from the rationale above.