Count me in - I've had problems where the sound in my DT50 fades out to nuttin. A reboot of the DT50 didn't help, so I rebooted my HD500, that didn't help. I had to shut both off completely and boot both back up, then it worked. Very weird.
I had another weird thing happen at church this Sunday - plugged everything up, then I noticed the screen on the HD500 would periodically display the amp settings screen, then go back to the main display, then back again, forever. I noticed the drive setting was maxed out. It was as if someone were cranking the drive knob up all the way behind the curtain where my amp was! I didn't have time to trouble-shoot, so I just went direct to the house. Later at home I realized what was going on, so I turned the drive knob to a different value on the DT and the problem disappeared! How strange. Whatever position it was in was causing it to send continuous MIDI messages back to the HD. Sheesh.
