Well, I use some DSP effect with my FW-modded Sony WM1Z with the MEST.
The one I use is their DSEE HX AI; it does nothing with files that are already hi-res, but improves the sound of 16-bit FLAC files a bit and can drastically improve the sound of some mp3 files, especially the lower-res ones, all using artificial intelligence-based algorithms and stuff to make the file sound more like a hi-res file.
I've got a bunch of Chinese pop music tracks, taken from friends years ago.
I've yet to figure out any way of finding them in FLAC format, let alone hi-res (if they are even available anywhere in that format!).
If anyone knows, please PM me. Seriously, I'm begging you, haha
But yeah, some of them are as low as 128kbps mp3
But thank heavens for the DSEE HX AI function; it opens up the soundstage and separation, brings clarity, smooths away the roughness and brings out the fine details too.
It's transformative, especially so on the MEST.
I haven't noticed any deleterious effects or weirdness in the soundstage.
One can imagine that by now Sony very much know what they're doing in this regards.
This might be a different experience to those using such effects on other DAPs or mobile phones though.
But yeah, that's been my experience so far; hope this helps!