I created some new ambient sounds with the HD500 and made a demo video:
Here are the tones: http://andy.kapsi.fi/line6/AnttiLoponen-Line6HD500-Ambient-patches.zip
Mostly it's just Auto Volume Delay with long feedback, masses of reverb and compression with various effects for spices. I control the feedback of the delay with the expression pedal to keep chords ringing as long as I wish, and to turn it down for switching chords.