By doing those steps, the ReplayGain would work only for the tracks you have scanned and written the gain level tags. ReplayGain aims at -14 dB relative to full scale, so this means the target loudness would be -14 dB of your X1 "volume capability". If you do this to all the tracks, then all the songs would be volume averaged between them.
To answer your last question, you could increase the levels of a specific track by modifying the signal's amplitude (using an audio editor). I highly recommend you not to do it though because you will end up distorting the song. It is one thing to amplify the volume using an amplifier that works with an analog signal (voltage through time) but changing a digital signal by software would result in a bad digital to analog conversion. You can try it and see
. The reason ReplayGain target loudness is -14dB of full scale is because it leaves a margin of 14 dB for allowing transient audio peaks to be reproduced without damaging the audio signal.
ReplayGain actually levels the songs at -14 dB in order to give some headroom for quick transitions and peaks as i said above
the only thing i don't like about it, is that since it leaves that headroom, the player will play the songs in a lower volume than it would without the ReplayGain function, limiting the max volume reproduced by the player. For the X1 that is not a problem though, it has plenty of power.