While I wait for arrival, can someone tell me what cable to buy for HD600 & HD800 that will plug into this?
At the front its XLR & what looks like RCA?
You need a different cable for each headphone.
The headphone outputs are 4 pin xlr (balanced) and TRS (single end).
I have the cable you mention and it certainly does the job for HD600. Sonically it is exactly as the HD650 cable.
By far less expensive is to reterminate yourself the default cables of your headphones. You can even buy the parts from Audio gd (http://www.audio-gd.com/Pro/Headphoneamp/Phoenix/ModifyEN.htm) and just borrow a soldering iron and a multimeter for half an hour.