we can use a simplified model ...
what is sound? frequencies translated in the air
what is a tonal difference? a difference in that frequencies
if the amplifier have a flat frequency response, there will be no tonal difference with other flat frequency response amplifier
so, if you can hear a tonal difference, is everything else (acoustics, speakers, gain, ..., mental bias) than a real amplifier difference.
i think is very simple ... and we know based on measureaments that the Fosi and the Cambridge have a flat frequency response.