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://forum.xda-developers.com/show....php?t=2365506