A secondary volume control setting would be desirable so that SB2 can be limited
to a lower volume range, for use with standalone amplifiers. This could be done
one of two ways:

1) In the server, add an additional attenuation setting which applies to the
main volume control.

2) Add a second "preamp" volume control which only applies to the DAC. The 
PCM1748 has built-in volume attenuation capability which we're not currently
using. By taking advantage of this, we could effectively let the DAC be set to a
lower range while still applying the full range to the s/pdif output. This
method may also give better SNR and dynamic range.

#1 is an easy quick fix, but #2 would be better.