Of course you can adjust the volume. What you cannot adjust for headphones is the "Optimizing the Output Level" (Pg 19 & 20 of the manual) This apparently is only for the line outputs on the rear.
"Optimizing the Output Level
Best performance is obtained when operating the Wadia 121Decoding Computer
Volume Control near the top of its range. If needed, the maximum output level
of your Wadia 121Decoding Computer can be adjusted to match the overall sensitivity
of your system so that critical listening will take place with the volume
control operating near the top of its range. Critical listening should be done when
the 4th or higher LED is lit. The maximum out level of the Wadia 121Decoding
Computer is adjustable by means of a series of IR commands issued from the
Wadia remote control. The Wadia 121Decoding Computer output level is factory
set to accommodate the most common range of system sensitivity. If you find
that your typical volume level during critical listening is below the 3rd LED on
the LED display, it will be advantageous to use a different setting"
got it, system gain type adjustment, thanks