0.1 to 27v is 48.63dB gain
1.0 to 54v is 34.65dB gain
Interesting results. The non-linearity is a bit of a head scratcher. The only thing that comes to mind is, is it possible the amplifier is clipping with the 1.0v input? But even that doesn't make a whole lot of sense, given the amps specifications indicate it's maximum voltage gain is 34.2dB, pretty much spot on with your 1.0v input measurement. Hmmm.