Did you take the motherboard from one of the non-working systems and everything else from a working system and see if it boots? If it does, then it's not the motherboard, if it doesn't then it likely is the motherboard.
In an extreme case such as yours, after you try a motherboard swap, to troubleshoot a non-functioning board, I would take a mother board out of the case and just attach a power supply only and see what indications you get: BIOS LEDs and audio beeps. Use the manuals to decipher what the indications are. Then add memory and repeat, then add video card and repeat, then add monitor and repeat, then add keyboard and repeat and so on until you get an indication of what the issue is.
It's very unusual to lose six systems at once. It indicates a possible electrical surge that could have damaged the motherboard or some other component. Also try a different electrical outlet and surge suppressor. Leave no stone unturned.
Good luck and best regards.