I think I remember some unwanted behaviour with my old 3GS, but it wasn't the volume bar not disappearing. When I turned off the HP-P1, the 3GS continued to play music through the internal speaker. A perfect match is the iPod Touch 2G, no problems at all.
Add: I think I'll wait until I have another DAC/Amp and can let the HP-P1 journey to Fostex for latest firmware. Seems the only way as I don't think anybody in Australia has the ability to update firmware on it.
Edited by AJHeadfi - 6/28/13 at 8:44pm