The Metrum DACs do not have a conventional preamp section. The potentiometer adjusts the reference voltage which in turn affects the voltage output thereby adjusting the amplifier output and the system volume. Steve Nugent's Empirical Audio DACs do it the same way.
My preamp was custom made by Steve McCormack of SMc Audio from the TLC-1 platform but highly modified and improved so the result is very similar to his VRE-1 preamp. Mine is set for unity gain while some of the VRE-1s utilize 6dB gain through their Lundahl transformers. The volume control was an expensive Shallco switch with Audio Note Tantalum resistors. Steve graciously instructed me how to unwire the Shallco switch so it is out of the circuit but two resistors were needed to replace the switch and I chose to use two very large and expensive Audio Note Silver Tantalum resistors. I am very happy with the result and the Adagio through the buffer IMO beats the Pavane through the preamp with volume control.
My preamp was custom made by Steve McCormack of SMc Audio from the TLC-1 platform but highly modified and improved so the result is very similar to his VRE-1 preamp. Mine is set for unity gain while some of the VRE-1s utilize 6dB gain through their Lundahl transformers. The volume control was an expensive Shallco switch with Audio Note Tantalum resistors. Steve graciously instructed me how to unwire the Shallco switch so it is out of the circuit but two resistors were needed to replace the switch and I chose to use two very large and expensive Audio Note Silver Tantalum resistors. I am very happy with the result and the Adagio through the buffer IMO beats the Pavane through the preamp with volume control.