Thank you but about what checkbox you're talking about? I use a portable player FiiO X5-II not an IPhone and looks like that it has not replaygain support.
ah ok, then yes you are one of the few that would want to do this (bearing in mind that the dynamics could suffer a bit). I googled your FiiO and found a forum article ... seems sort of strange it doesn't support any volume metadata. But looks like they are looking into it in a future firmware update.
Anyway,
after you have all your music analyzed
You can try this after tracks are selected
on the menu go Tools =>Advanced => Convert Format
choose your encoder, click options and use the wizard like in this screen shot, click the "box" I was talking about and choose the DSPs you want.
Be careful to not overwrite your original files. I've read that it is better to use 32bit at least (I think the automatic default goes as high as the encoder you choose will allow not sure -- I'd check after doing a couple.) -- apparently this can help avoid clipping if I understood correctly.
If Fiio adds some kind of replay gain or R128 volume control, you might want to reconvert without the DSPs, though. You might want to save a playlist of the tracks you have converted to make this easier. I'd also try a small batch of tracks first as a test to hear it on the player. Depending on what DSPs you are applying - if any other than peak volume leveling - might need some tweaking