I have been using it since this morning, and it is awesome. Great job!
I want some clarifications on what 5 does.
And 6, please.
Cheers!

-HK sends
I have been using it since this morning, and it is awesome. Great job!
I want some clarifications on what 5 does.
Replaygain seems to be working fine for the most part, but it doesn't seem to be applying to mp3 files, at least when switching back and forth between them and flac.
Hah, thanks. Yeah, my wallet has been suffering lately, go figure.
As for replaygain, I just tested it with 2 copies of the same file, one with replaygain data and one without. The non-replaygain version was noticeably louder, so it looks like the X5 just applies existing tag data rather than analyzing on the fly.
2016-01-30
Beta firmware version 1.27 for the X5 2nd gen now available!
Download link: https://www.dropbox.com/s/ugs1xw2o72a8byl/X5%202nd%20gen%20FW1.27beta.zip?dl=0
After downloading the update, please unpack (unZIP) the package and read the update instructions contained within.
Please note that all firmware posted in this thread are beta firmware--please post in this dedicated thread if you have any issues with these versions, and if you encounter serious problems, consider flashing back to a stable release.
Stable firmware can be found here: http://www.head-fi.org/t/776861/official-x5-2nd-gen-stable-firmware-thread
The following changes and improvements have been made to FW1.27beta compared to FW1.21beta:
1. Thai and Russian language options added;
2. Added support for ReplayGain;
3. Increased USB storage transfer speeds (≥8MB/s);
4. Added Recently Played playlist (via Play by Category);
5. Added DAC output reconstruction lowpass filter options (steep and gentle slope)
6. Added DoP and D2P options to S/PDIF output;
7. Added Line Output level options (fixed or variable level);
8. Optimized UI operation and speed;
9. Fixed gapless playback problems;
10. Fixed issue with hidden files stored by a Mac OS computer accessing the cards on the X5 2nd gen;
11. Fixed issue where changes to play mode in Play Settings may not be immediately registered;
12. Added play mode: play single track and stop
13. Added option to delete a track (outside of Now Playing);
14. Changed startup animation
15. Added option to manually clear media library database file (via System Settings);
16. USB OTG function (which had never been compatible with many USB devices) has been disabled to improve device detection in storage and DAC modes by the computer;
17. Miscellaneous bug fixes.
Can't comment on the other features, but yes, replaygain is basically volume normalizing. You apply replaygain metadata (like tag information, nothing that actually changes the file permanently) through an external program like foobar. Once that's done, if replaygain is enabled it adjusts the volume of a file upwards or downwards as needed to achieve a certain loudness. So yes, you have to pre-tag your music, but you don't have to re-rip it.
Thanks for the info about replaygain.
I found Foobar2000 has quite a complete online manual at this link:
http://eolindel.free.fr/foobar0.9/Replaygain.php
I don't think I'm going to do much with this feature anytime soon.
It also appears that the two digital filter slopes are for DAC mode. [ 5. Added DAC output reconstruction lowpass filter options (steep and gentle slope)]
Missed that in Joe's original list of changes.
I like this new fw a lot so far! (I found a few *very* minor "typos" which I corrected using the custom mod tools.)
Charlie
I just deleted all replaygain data from all my files. It seemed to make everything rather dull. Why would scan per track result in -11 ~ -13 db for all my songs? I like my music this way.
Seems like that digital filter affects SQ, or at least I think so, and setting it to steep makes bass tighter, while the other prolongs bass notes. Maybe. I am not sure.