Not directly answering your question here, but if you're using variable output on the sonos, the biggest gain (no pun intended) you'll get is to switch it to fixed output. If that requires a preamp, ok, but that variable out is where a huge amount of the veiling is on the sonos, IME.
So preamp, DAC, Cullen would be my suggestion.
So preamp, DAC, Cullen would be my suggestion.