On the HD500's looper, the manual describes the overdub parameter:

Use Multi-function Knob 2 to adjust the level at which all Overdubs are recorded. This level does not affect the level of your original Loop recording.

However, upon testing, it seems the overdub parameter works exactly like it does on the Mseries:

To clarify, if your Overdub Level is set to 90%, each time your loop begins a new overdub its volume will be reduced by 10%, sounding quieter and quieter with each overdub pass.

From what I can tell, the 2nd way is actually a lot more musical. If this can be confirmed, then the HD's manual is wrong. Which is a shame because it kept me from playing with this parameter for a year, thinking that I couldn't do anything with quieter overdubs if the original loop didn't fade along with them.

So, can anyone else test their loopers and see if the overdub parameter affects the initial loop volume? It certainly seemed to on mine- but only when it was in overdub mode, not when the loop was just playing back. Would still love to see this parameter and loop volume level controlled by the exp pedal though.