now i'm on custom rom, S-OFF, Unlocked, Rooted and hboot downgraded (1.44) so at this point i can flash the ruu.exe for my htc one (x.401.x.x) but still i don't know if i'll manage to get it back to the Cosmote rom (had some cool programs installed on it)..
if you want to go back to stock and you have s-off and 1.44 hboot then the only thing you need to do is to download and run the 1.28.401.7 ruu.exe from htc1guru.com
it will wipe your phone and restore to stock rom and stock recovery. you will still be s-off. Just don't forget that if you want to remove the tempered flag, you should do it before the ruu