Circling around a bit, what's the advantage of having a separate dac for desktop speakers and headphones? While an audio interface would better utilize my external mic (Samson Q2U), using it via USB works just fine for my needs. For less cost than a D10b (HP) + a dac with a volume knob (e.g., Motu M2 - powered speakers) + L50 (HP), I could buy the L50 + DO100 and with a flip of a toggle switch back and forth from HP to speakers, each volume controlled with either the L50 or DO100 volume knobs.
Since apparently the DO100 does not have DAC or preamp modes, and therefore controlling volume via the L50 is not optimal, the best solution is to control volume with the digital DO100 volume knob. What I don't quite understand is with my current SE inputs to speakers, why I must also use SE connection from DAC to AMP. Would it be possible to make both SE AND Balanced connections (cabling both RCA and XLR/TRS) from DAC to AMP so that when I use my HP, I would switch the L50 to BAL and use the added power of a balanced connection between DAC and AMP, whereas when I turn the L50 to OFF and use the passthrough of the L50, the SE connection would be intact from DAC to speakers? Thus, with a switch of the L50 toggle from BAL to OFF, I could control either HP or speakers (not both together) with the DO100 volume knob. It seems to me that this is a simpler solution than buying a second dac with volume control and using three devices (2 dacs, 1 amp) to run both HP and speakers.
And once I buy speakers with balanced inputs, I could remove the SE connection between the DO100 and L50.