The line-in gain control on my mid-2011 iMac (in Audio MIDI Setup) allows the gain to be increased to a maximum of +12 dB, requiring around 700 mV RMS for full ADC output. According to the Apple support site, the line-in gain can be adjusted up to +30 dB, allowing full output with an input voltage of only 63 mV RMS (iMac (Mid 2011): External features, ports, and connectors - Apple Support). Is there some way I can increase my gain to +30 dB?
Interestingly, my early-2009 Mac Mini allows the gain to be set to +30 dB. Both are running OS X 10.9.5.