...or I guess you could just leave the knob up and use whatever source you're using as a preamp. Although then you get into the question of whether or not the digital attenuation degrades the sound. (Short answer -- it depends on what algorithm they're using -- if it's one of the better companies, then it's probably fine.)
If you're doing it from within a computer, you could try just setting it to 50% or 25%, and using the knob on the Bryston in the upper half of its travel to adjust volume. That way you're only using 1 or 2 bits of information.
EDIT: That said, all of my suggestions were compromises. Especially the preamp -- not sure adding an extra component to your system is going to degrade your sound any more or less than using digital attenuation, even Windows digital attenuation.