When dealing with these situations, I often like to raise the unamplified portion, yet still maintain some (but not all) dynamic range. During listening, I just don't find it natural to hear an unamplified song at the same level as the amplified tracks - not to mention raising the volume on an unamplified track also raises the noise along with the signal.
I usually use a volume envelope to fade in the volume increase, keep the volume increase level during the song, and then fade it back down after the song's over (in time to not get blown out by the applause). How much? I usually just do it by ear, so it's quiet relative to the amplified portions, but still listenable without turning up the volume on playback.