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.
Yeah, adding a preamp for volume control seems like it would only make sense in the context of a bigger system. Like, say, working the CMA800R monoblocks into a larger speaker based rig. But in that case one likely already has a pre anyway.
There does seem to be a correlation between DACs with volume control and those with integrated headphone stages. But don't let that shorten your list - some great DACs have headphone amps on board, but you can just ignore that part if you want. I'd say the new Benchmark DAC 2 is worth looking at, the Anedio D2, Wadia 121, Yulong DA8, Resonessence Labs Invicta, and the Grace M903 (among many others). Models with volume control but no headphone stage include the Resonessence Invicta Mirus, NuForce DAC-80, MSB Analog, SOtM sDP-1000, Onkyo P3000R, NAD M51, the Perreaux Audient DP32..... and many more I'm sure.