You'll need to wire the headphones as balanced as a safety measure. This is because
some amplifiers
might have some kind of problem if the ground is shared (if the L+ and R+ ever meet, say bye-bye to any amp), without feeding the 4-channel* signal into an L-Pad or other such circuit first, unless you're sure that that particular model doesn't have such issues (ie if anyone's used that same amp in that manner). Conventional headphones, particularly a single-entry (ie cable goes into left cup) design, uses a common ground with independent L+ and R+ conductors; even a dual entry headphone like Grados and some Sennheisers have independent conductors all the way to the plug, but once in there the GND are soldered into the same terminal on the TRS plug (obviously, a balanced HD800 with a 4-pin XLR plug has them separated).
So basically you rewire in one of two ways:
1. Rewire the headphones and terminate the amp end of the cables with banana/spade plugs (depending on what your amp can use, but I'm biased for banana plugs if it can use them)
2. Rewire the headphones to end in a male XLR plug; then make an adapter for the speaker amp made of cables with banana/spade plugs on the amp end then a female XLR on the other end. You then plug in the headphone's male XLR into that female XLR. Make sure both are 4-pin and you solder them appropriately.
One thing to note though, AFAIK Class D amplifiers have some kind of issue driving high impedance loads (can't remember if THD at some point starts piling on faster, or if it has to do with damping factor). I remember there was someone who was able to wire a T-amp and the results were good considering the price of the amp, but all the same if you're going to use anything above 100ohms don't expect much; also, you're probably going to have less than 100mW into 300ohms.
*By "channel" here I mean the conductors as when people describe what configuration their Beta22 is in, not channel as in stereo or surround
Thanks, but that isn't specific enough to even tell me what to search for exactly. What do you mean by at the free end? How do I do this without it being DIY? I just want to buy the cheapest things I need to get the job done.
Googled and found these links:
"Spade plug"
http://www.amazon.com/Sewell-Premium-Degree-Connector-Pieces/dp/B0050CEA40
http://www.speakerrepair.com/ebaypics/new-spade-plate-500.jpg
"Banana plug"
http://www.nakamichiplug.com/product-2503.html
https://www.youtube.com/watch?v=ybRqeCkgXWs
And from my post above:
"Male 4-pin XLR plug"
http://www.amazon.com/Neutrik-NC4MX-Connector-Nickel-4-Pin/dp/B002EDO2K0
"Female 4-pin XLR plug"
http://www.bhphotovideo.com/c/product/748113-REG/Neutrik_NC4FX_B_NC4FX_B_4_Pin_XLR_Female.html