Most headphone amps can double as preamps, so putting a dedicated preamp between the DAC and a pair of monoblock CMA800R amps just for the volume control seems a little redundant, and most likely will add its own color and/or shortcomings into the signal train.
Seems like you ideally need a DAC with just enough analog pre-amp capability for volume control to run the dual CMA800R setup. Since none of my current desktop or home dedicated DACs have volume adjustment capability either via local knob or remote, I wonder just how prevalent such DACs with volume control are these days, or are most of these type products offered as combo DAC/(HP) amps?
It might be a worthwhile exercise here to enumerate some of the current pure(?) DACs with volume control around that might be held in good esteem via user and/or reviewer recommendations.
To start, and I realize this might be perceived as overkill, the MSB Analog DAC has volume control as an option.