I'm rather confused as to why there is talk about putting the external DAC BEFORE the Lyngdorf, instead of AFTER the Lyngdorf?
The Lyngdorf 2170 & 3400 has a digital coax output.
On the 2170 at least, the chain you could or should be running is:
Streamer -> digital input of Lyngdorf -> digital output of Lyngdorf (RoomPerfect™ applied) -> digital input of DAC -> analogue output of DAC -> analogue input of Power Amp -> speakers
In this chain you have full signal from the DAC to power amp, and you digitally control the volume on the Lyngdorf as normal.
Yes there are usually concerns about bit depth degradation from digitally controlling the volume, but I'm pretty sure their Equibit technology goes a long way to diminish the issue.
The important thing is that the analogue gain is at full (2v or 4v) strength from the DAC itself. You just put the volume control in the digital domain before the DAC in this case, which always happens anyway with the Lyngdorf.
Someone should try this solution and see what happens.