Thanks to johnwmclean and Apuresound for the info.
Thanks for all the responses, Heli from apuresound has been most helpful in providing information I needed, here are some tips for other people who are interested or need to terminate the stock HD800 cable over to (2) 3 pin XLRS:
As for tips in re-terminating I watched Alex do 2 pairs so far I would
say overall it is straight forward. Watch out for when you remove the
left and right rubber coverings since this cable is shielded you don't
want to cut all the way through the shield. The 2 wires behind the
shield are your +/- for each side. The red side is left + and green is
right +. The other wires are the return wires. You can wire up the
shield to pin 1 in the XLR connector so it works as an active shield as
long as your amp is setup for it. You may be able to remove the thin
insulation off of the wires either by a lighter or a very hot soldering
iron, but this isn't ideal since it melts and burns which will leave a
residue on the wires. The insulation is thin enough to where if you
score it on one or two sides you should just be able to peal it off with
enough force. The main thing you need to work out if how to make sure
you won't pull the cable out of the XLR connector since the wires will
not be of much help holding it in place. The process is not hard or very
especially if you have the proper tools. Just be careful and don't rush
it. If you mess up you will have to simply cut further back on the cable.
I’ve been really impressed with apuresounds responsiveness for the help I needed, considering there was nothing in it for them as a company. Goes to show you there are still companies around that have great work ethics.
So impressed am I that I’m in the process of ordering one of there V3 cables for the HD800, once the connectors are available should be around 2nd July