S-ON is a problem. RUU and firmware flashing should only be done with S-OFF.
What specific latest ATT RUU? That doesn't make much sense because the latest 4.3 leaked ruu.zip for ATT should have hboot 1.55.
If you flashed an ATT firmware that has you at hboot 1.54 and you are S-ON, then you can no longer run any of the RUU's that has a lower firmware base. Not sure what to advise you.
You could always download the latest 4.3 ruu.zip for att here:
Then extract out the android-info.txt and change the ATT CID to your CID. Then re-insert the text file into the zip and flash from the guide in the above post.
You will be then running 4.3 ATT rom. It should be able to flash with s-on because it is the signed images in the RUU and you should be lower in firmware.
You should be able to unlock bootloader again if needed and install a custom recovery and then install a rom normally or just keep running the 4.3 leake until they can find a way to s-off with hboot 1.54.