I know this is an old post, but I am also having this ongoing issue on both Studio One 4 and 5, and none of the other forum threads have suggested anything that works. My exported songs are much lower in volume / level than what's being played back in Studio One.
As an example, if I create a tone generator track with a 1kHz tone, and play it back, the level is -28dB using a level meter post master fader. If I export the track to a WAV (or any other format), it is really quiet. If I then drag the WAV file back into Studio One 5 as an audio track, the level is now -40dB. Any idea what's going on here anyone?