Most transparent power amp

First Watt F5. Ruthless to a fault. 

@georgehifi 
There are "passive" amps. The First Watt F4 is an impedance converting amp with exactly 0 dB of gain that only serves the purpose of delivering the current so that a line level signal can be fed to the speakers. 

@almarg 
In a strict sense, yes, but passive preamps utilize active components to buffer the input, volume, or both, so that the variation in impedance by the volume control doesn't reshape the response. Generally, so long as the circuit's goal is impedance conversion, it's called passive because it's doing the job of a passive component, namely a transformer. 

Not talking about the F7.