for OTA you must have stock rom, recovery, kernel e no root but you can flash hboot and firmware manually via fastboot if S-OFF
p.s. sorry for my english
The ROM and the firmware are seperate.
The firmware usually contains hboot.
Custom ROMs do not include firmware.
The OTA includes firmware as well as a ROM patch and must be flashed together with ROM and Recovery both stock.
You can update the firmware manually.
If you want to flash a modified firmware (e.g., recovery & kernel removed), or a firmware for a different CID/MID/Part number you need S-Off.
restoring a nandroid will not update your firmware or hboot
theirs 2 ways to update firmware the way HTC does it is with a RUU or OTA update
But since we have s-off you can simply get s-off and flash any firmware you like
look here >> http://xdaforums.com/showthread.php?t=2365506