Ok I am trying to determine what is the best voltage value I should use for a AC/DC power supply for my rpi2+Amp+Volumio HiFi renderer. As you mentioned I may get a ~3db increase in vol for the same input signal if I run the renderer at 18v over 12v. Also 18\v power supplies sometimes have overvoltages when the current load is very low and this may damage the renderer. Also the Amp+ voltage regulator is working al little more at 18V attenuating the voltage thereby generating a little more waste heat.
So if you were building your own rpi2+Amp+ renderer and had infinite funds what voltage AC/DC power supply would you use and why?