Hi,
hope my procedure may help you solving the booting problem :
1) Parallella board have 2 green leds + network port leds (4 led 3 green 1 orange) = if the 2 green leds on the board are not on, but only one , should mean that the boot process is not going well (maybe some mistake in bit.bin file)
2)when you dump the .img file on the sd card the "bs:4M" option looks like a must for me (without it the sd wasn't printed correctly.
3) i dumped first the headless version then overwrite the hdmi version without formatting (that may be useless because dd should overwrite the data but i'm not sure)
4) I dig in the github to retrieve the "correct" (not sure it is correct) .bit.bin file and overwrite / rename on the fat BOOT partition. (https://github.com/parallella/parallella-fpga)
5) i did overwrite the .dtb file from the hdmi version (should be the same but you can check with md5sum)
6) you can test if the board is working by checking the network device list of your router and by checking the ethernet port on parallella board if only the green let is blinking you r not online, if the orange is blinking you should be able to ssh the board. (headless image should run on hdmi version allowing you to access via ssh and (still headless) may work better than hdmi version)
I'm on a 7020 hdmi so the result may not be the same
hope to pointed you in a direction to solve your problem.
Cheers
A