There aren't a ton of headphones that need 8,000mW of power, but if you'd like to try headphones on the Kaiju, and you have the DC heater board, give the AKG K-1000's, Hifiman HE-6, or other power hungry headphones a shot.
HD-650's will work when powered by the Kaiju, and the best impedance setting will be the 2 Ohm output.
-PB