Suppose a system comprises a DAC with preamp and on-board analog volume control (generally I suspect analog volume has higher performance potential vs. digital volume control).
Let’s further suppose one has a separate high performance pure analog preamp w/high performance power cable and analog IC.
I suspect that the higher is a power amp’s input impedance, the more likely would listeners prefer overall performance with the above described DAC source direct to power amp, and the less likely would the above described preamp improve performance.
It may not be a coincidence that Bricasti’s M21 is a superior DAC with analog volume control (and separate DSD and multi-bit ladder type DACs), Bricasti does not make a separate preamp, and Bricasti’s power amps have very high input impedance: Bricasti specifies M15 input impedance of 200k ohm; Atkinson measured 170k ohm IIRC, which John called "usefully high."
There’s another potential fly in this ointment: subwoofer power amps, which always or often have very lower input impedance in the range of 10k-12k ohm. Systems with one or two sub amps are more likely to need an active high performance preamp.
If one wants a system with the flexibility to audition any power amp, again, a preamp would seem to be mandatory.
