I'm in the process of installing the custom kernel - very exciting 
Big thanks to @jkbuha and @prispewnic for undertaking such an ambitious project and sharing it!
I'm unsure which kernel version to install though - my main concern is battery life... Is there an significant difference in battery life for the various 1416MHz, 1704MHz and 2016MHz kernels? Anyway, I'm starting with the 1704 one, hoping that throttling keeps the power usage low most of the time. It's quick to swap kernels later anyyway, thanks to the nice installer.
There's one thing I'm curious about though. I hear people talking about sound differences in the various audioflinger versions (and also for Fidelizer). Does the audio transport (file > player > OS > DAC) of Android apply any hidden/forced filtering, dynamic processing or similar behind the scenes, apart from number (sample) format conversion and volume scaling, that varies between versions as well? For only a single sound source, sample rate conversion shouldn't be needed for rates supported for the hardware. If so, is that to force some kind of "sound signature" on the users by the manufacturer modifying OS components to do just that?
BTW, my player came with 1.2.4 pre installed, a version not available from the FiiO site(!), and it has an RK3188 (not the T version).
Edit: One more Q:
After installing TWRP and attempting to upgrade the firmware with the Technical support app using local "X5-ota-1.2.5.zip" [1], the player boots into TWRP and selecting install and picking the ota archive ends in an error. This means I had to use the "X5III firmware upgrade tool FW1.2.3.zip" and then install the local "X5-ota-1.2.5.zip" afterwards (I can't find a 1.2.5. "update.img" file anywhere...). Is there a way of still installing a local ota file with TWRP installed, or simply keeping the browser?
[1] The reason for reinstalling 1.2.5 after the 3.41 kernel was installed was because i chose the "average" app removal which got rid of the browser, and I didn't find an obvious way of getting it back. So it was faster & simpler to just install a fresh 1.2.5 (1.2.3 first...) and then the 3.41 kernel again. Yeah probably very clumsy - this is my first Android device so I'm kinda clueless
Oh, and one thing that surprised me, was that on the install of 1.2.3 there was a lot of apps there. On the original 1.2.5 install, there was only half a screen full of apps, and with 1.2.3 there was one full screen and a few more, most of which seemed useless. After the second 1.2.5 install all the apps from 1.2.3 and 1.2.5 seem to have accumulated. Oh well, I'll just do a round of disable/uninstall for most of it.

Big thanks to @jkbuha and @prispewnic for undertaking such an ambitious project and sharing it!
I'm unsure which kernel version to install though - my main concern is battery life... Is there an significant difference in battery life for the various 1416MHz, 1704MHz and 2016MHz kernels? Anyway, I'm starting with the 1704 one, hoping that throttling keeps the power usage low most of the time. It's quick to swap kernels later anyyway, thanks to the nice installer.
There's one thing I'm curious about though. I hear people talking about sound differences in the various audioflinger versions (and also for Fidelizer). Does the audio transport (file > player > OS > DAC) of Android apply any hidden/forced filtering, dynamic processing or similar behind the scenes, apart from number (sample) format conversion and volume scaling, that varies between versions as well? For only a single sound source, sample rate conversion shouldn't be needed for rates supported for the hardware. If so, is that to force some kind of "sound signature" on the users by the manufacturer modifying OS components to do just that?
BTW, my player came with 1.2.4 pre installed, a version not available from the FiiO site(!), and it has an RK3188 (not the T version).
Edit: One more Q:
After installing TWRP and attempting to upgrade the firmware with the Technical support app using local "X5-ota-1.2.5.zip" [1], the player boots into TWRP and selecting install and picking the ota archive ends in an error. This means I had to use the "X5III firmware upgrade tool FW1.2.3.zip" and then install the local "X5-ota-1.2.5.zip" afterwards (I can't find a 1.2.5. "update.img" file anywhere...). Is there a way of still installing a local ota file with TWRP installed, or simply keeping the browser?
[1] The reason for reinstalling 1.2.5 after the 3.41 kernel was installed was because i chose the "average" app removal which got rid of the browser, and I didn't find an obvious way of getting it back. So it was faster & simpler to just install a fresh 1.2.5 (1.2.3 first...) and then the 3.41 kernel again. Yeah probably very clumsy - this is my first Android device so I'm kinda clueless

Oh, and one thing that surprised me, was that on the install of 1.2.3 there was a lot of apps there. On the original 1.2.5 install, there was only half a screen full of apps, and with 1.2.3 there was one full screen and a few more, most of which seemed useless. After the second 1.2.5 install all the apps from 1.2.3 and 1.2.5 seem to have accumulated. Oh well, I'll just do a round of disable/uninstall for most of it.
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