As above. 

The 2 DACs I have already haa digital volume control which I believe are provided by the AKM/ESS chip.

Thanks

The ESS has a noise floor of -135db, I don’t know if any analog volume controls can compete.

It depends. Usually you want the attenuation to be at a later stage of the signal chain when possible, since the noise of the previous stage (DAC) will be amplified by the gain of the later stage (headphone amp). You can argue that the DAC's intrinsic noise is low enough that even if amplified by the full gain of the later stage (headphone amp), it is still well below the intrinsic noise of the later stage, in which case it doesn't make much of difference implementing the attenuation at either places.

There are also practical concerns. Using a passive attenuator such as a volume pot increases the source impedance of the later stage, which often increases its noise and in some cases, affects its frequency response and linearity (more distortion). Many DACs with brilliant SINAD numbers employ distortion compensation. They are tuned for the lowest measured distortion at 0dBFS input. As soon as you reduce the (digital) input level, the harmonic profile changes quite significantly. The result of the above is that the system may 'sound' different with digital volume control vs analog volume control.     

Last but not least, one's ears are the most valuable device in the signal chain. Personally I'd rather have an old-school volume pot as close to the headphone amp/power amp as possible as the 'emergency shutoff valve'. 

Edited September 25, 20232 yr by simmconn
clarifications

On 9/24/2023 at 1:08 PM, johnwmclean said:

The ESS has a noise floor of -135db, I don’t know if any analog volume controls can compete.

What about impedance matching?