Although I live in the US, I have the European (global) firmware, (phone purchased from Gearbest). It is my understanding that the EU limits the available volume to some extent. The headphone extension trick works well on my phone, allowing access to high gain mode.
The gain boost in the settings menu is due to the switch between the Snapdragon 617 DAC (standard) and the higher powered AKM 4962 DAC (super) .
I am able to go from stupidly loud (for me, we all have different volume tolerances, may not be loud enough for you, lol), to insanely loud using the adapter trick paired with the AKM 4962.
Best Buy has the US GSM version in gold for $159, add $20 for gray. That is lower than the current Gearbest price for the gold.