Too quiet at Main out - what to do?

Digital gain is digital gain.  Hopefully your file is still 24 bit you so have a low enough noise floor to boost with relative impunity.  A limiter is usually the best route since it is easy to manipulate and monitor the results.  Or, if you prefer, boost a bit with a compressor to gently even out the peaks a couple of dB and finish off with a limiter.  

Using compression/limiting won't just add gain it will change the RMS vs. peak levels. Unless you use a compressor with it's ratio set 1:1 of course.

The easiest thing is just to turn it all up. Either using the master fader, master gain trim, an empty fx chain in the master pro-channel (which will still work as a volume control), or route everything to a pre-master bus and do the same kind of volume-raising on the bus.

Another easy option is to import the unintentionally limited final stereo wave into a track in a new project and simply normalise it to -0.3dB.

If everything sounds OK with the volume increased, job done. If turning everything up reveals the track would benefit from limiting/compression then add the compression.

If the inadvertent limiting doesn't work for the song, just load the project, remove the offending limiter and see what the volume's like without it then re-mix as required.

bitflipper
If the exported file peaks at -14.2 dB but otherwise sounds fine, then you can raise it by 14 dB after the fact and it should still sound exactly the same, just louder.

Thanks guys for the input - what Bit says is exactly what I needed to know, I was just kind of not trusting that could be the truth.  And like @rumleymusic sais, "Digital gain is digital gain." - so just adding gain, even of 14db, does not change the quality, just makes it all louder.   I did just that, added gain at various stages, and was actually quite satisfied with the result.