I'm also selling my M17. I love the M17, but after I paired with a Cayin C9, I love the C9 sound more LOL. It just doesn't make sense for me to lug around the M17 if I'm using the C9 >90% of the time so going with a lighter option.
I finally kept the iFi ipower elite 15V .. after making some comparison with the standard FIIO external power supply, though it's almost impossible to make a real A/B comparison, I had the feeling the sound was maybe with something like faster with better kicks like an external THX Amp (THX887) I use usually for such target...
In real life, the M17 in enhanced audio mode with the standard is already very convincing, I am not sure it's not a "placebo" affect that I felt with a 4A power supply, the M17 could draw juice faster than with a simple 3A power supply.
I used 2 power hungry headphones like HEDDphone One and DCA Stealth.