However, a third machine COULD be brought into the equation but I'm reluctant to do so in case there is something nasty being passed from machine to machine.
I'd guess that you are beyond software problems now - once you are counting POST bleeps you are looking at something adrift from a hardware perspective (either faulty or incompatible - in your case the former as this setup previously worked nicely, unless you very recently added something or flashed some firmware?).
Wonder whether you have a component that is so faulty that it belew the original motherboard and has now taken the second one with it? Or maybe it is just coincidence?
Sorry can't offer further advice at this point - hardware's not my thing
RC