Thanks for the help !   The volume is already quite loud using the Clip alone but I like to have a small marging. I'll go for the Fiio E6, plus it seems to fit well with the Clip. I'll give a feedback soon.

Hello,    I just purchased an AKG K518 headphone. I like it so far but I have a little concern about the volume. It is quite low compared to my Sony XB500 and Klipsch S4 with of course the same volume setting on my Sansa Clip Plus (with Rockbox).    It's loud  at about -15/-5db but still...