The Looper in the HD500 is mono. Kind of a shame for a device with so many stereo effects.
My quick fix was to grab a Boss RC-3 and put it in the FX loop. I also have a Digitech Timebender in the loop before the RC-3. Now it’s very easy to get full stereo ambient effects. If I get more into looping using this method, I may swap out the RC-3 for something a little more involved.
ps – the fx send from the HD500 is a stereo send if you use a TRS to 2 x TS cable.
Obviously the board (if you can call it that) is kind of messy right now –
bob daspit 
At this point, I’ve pretty much ditched all the pedals – just using the HD500 by itself as an audio interface for my laptop.
My latest plan is to use the HD500 as a pedalboard with a real amp – some sort of mini head with an fx loop. I’ll give the “4 cable method” a try.