How can I wire up my headphones to an speaker amp's outputs

balanced 3-pin xlr configuration or 4-pin xlr? balanced 4-core 1/4'' jack with both the negatives soldered together to form a shared ground? what connector is your headphone?

if dual 3-pin xlr( i see this configuration very odd to be honest when coming to headphones) all you can do is get pair of neutrik female connectors online for few bucks and solder pair of bare speaker wire ends to 2 and 3 pins on the plug(2-positive, 3-ground/negative) and done.

4-pin XLR makes more sense with headphones to be honest but bit more hassle to solder bare wire to it cause make sure to do it correctly. you can use the AKG K1000 manual for reference use for wiring a 4-pin xlr plug.

for a headphone with 1/4'' jack, you can strip it and terminate it with 4-pin or dual 3-pin male xlr connectors and them make female xlr cables or can hook it directly to back of the speaaker outputs if you don't want to terminate a new connector to it or rewire the headphone. or can make either simple ghetto female 1/4'' TRS jack with both the negatives soldered to the ground terminal on the jack or can make a simple project box and parts from your local radioshack to make a female 1/4'' jack to speaker output box and run it that way. radioshack still sells project boxes and have all the materials.

for buying the connectors. i think they sell 3-pin xlr to speaker wire cables for speakers and so forth on amazon and local guitar centers but i i never saw 4-pin to bare wire cables before selling besides the ones on ebay for the K1000 if that's what your looking for. if you go with the 1/4'' to speaker outputs and decide to solder the two negative wires as a common ground make sure your amp is also common ground before doing anything or you will run into either killing the headphones or the amp or if lucky it'll just constantly be going into protection mode if it has one. hope this helps a bit.