x1 is the modem code that johnson used in his older custom firmwares that aren't on github, but on this forum as dropbox links.
x1 was generally only beneficial on vectoring lines and had stability issues for regular lines [or just performed horribly for some other guy on this forum], so it wasn't recommended to anyone without vectoring.
x6 is should be better than x1 and i am currently testing it [i tested x1 in the past] - i can't tell you if it's good yet, i have to wait until i'm 7 days uptime [i had line drops every 6 hours the past week due to external interference which is what led me to trying it. it didn't solve the issue but the last time i had interference was more than a day ago so maybe i would get 7 days uptime to know if x6 is stable. so far it's stable but specific people can get completely negative results so i would say that even then, it still depends].
what i recommend for now is getting the .bin without "x6" in its name because it contains the version "v" which is the original modem code that is "truly" compatible with the dsl driver which is d26a. x6 was taken from a newer ZyXEL modem with the dsl driver d26s so it may have the same issues as x1.
for now, unless you have a line with vectoring, get 1312-B10A-17-jumbo-tel-stats1.1-cmd.bin [modem code version "v"].
i personally chose one with "stats" in it since the program "DSLStats" seemed to consistently crash my router into a forced restart after a certain period of time whenever i used it. even if i used it once. the stats server doesn't take much or any processing from the device so it can't hurt. v17 signifies the 1312-B10A firmware version released by ZyXEL, before johnson added the other things. v26 is for 8924 and 8324 so it's not for your device.
i still telnet "ifconfig ppp1.1 mtu 1500" [i have pppoe 1492 mtu without the baby jumbo frames] on each ISP connection establishment because i still have to for some reason. it could be because i didn't reset the device to default after upgrading from v14 with jumbo frames by Olipro. if you don't use the device in bridge mode you might have to do that to the PPP link. maybe i misunderstood what johnson said though:
1508 MTU 'baby' jumbo frames
Changes to to the ethernet driver to allow > 1500 MTU ethernet frames & changes to the PTM network driver to bring the link up with an MTU of 1508 by default.
not a problem though.
here is what the SNR graph showed for x1:
https://imgur.com/a/xsJ6DEuat the last tones you see a curve downward. that did not happen on V which had a spiral upward.
so far into day 2, x6 doesn't have that problem. [i don't have vectoring] but, actually, i don't have DLM on my line. so i can experiment with anything and do whatever i want without consequences, unlike most people. you probably have DLM on your line, so i recommend sticking to V until i test this one. [its compatibility with the older dsl driver d26a, that is].
you can jump in on x6 right now but without vectoring that decision is questionable, as i still don't know how it performs. you can see in this thread that "one guy" who had horrible results from day 1 with x1:
https://forum.kitz.co.uk/index.php/topic,21683.msg376325.html#msg376325edit:
this post has three graphs, one of x1 and two of V showing what i mean about SNR graphs differences. [because it has a deeper meaning than just less downstream sync or whatever].
i attached x6 close up image and those last tones are pretty similar to V's as you can see [so far, as i can't tell what will happen by day 7]. V had a bit of an upward curve rather than straight but it's a very tiny difference in terms of SNR gained. i'll test V after 3 weeks with x6 or something like that. i'll also take in consideration the error rate differences but those can vary on their own. looking at overall rates over 3 weeks on both should be conclusive. other people could have very different results than mine though.