Not sure I still fully understand but anyway, there's no way around this, If he wants to use digital crossover, he'll need to have more than 2 channels of digital to analog conversion The Digital DSP solution that you suggested don't provide that, It don't work. The mini DSP solution that contain ADCs and DACs would work, but that is not what you proposed and he specifically said that he would like equal to RME specs and measurments, however if it matters or not is an other question. But in all case, there are no mini DSP solutions that would not render his RME DAC irrelevant.1) Of course the RME is a stereo only DAC, that's where this particular users issues start, selecting the wrong product for what he wants to accomplish (running a 2.x setup).
2) And of course miniDSP can provide the analog outputs he needs, and is what I first suggested (i.e. a tool that actually accomplishes his goal, which he said did not exist).
3) The user insisted he wanted to use the "better" DAC's that RME has. So I suggested the all digital miniDSP solution, and he is free to add whatever DAC's he wants to use (or use the other suggestion I made, use a analog/digital output miniDSP solution, run the mains off his RME DAC and miniDSP analog out to his subs).
Anyways time to move back on topic, and away from this pretty elementary way to get a 2.x system that includes the RME DAC the user wants to use. This Hypex product will never be what is needed if he is insistent on using his RME DAC (which doesn't even have analog inputs).
His best bet, in the case that he mentioned which is simply 1 USB source and no other, and the need for multi out, is as Sokei proposed is combining a software based DSP and crossover to a multi channel DAC. No miniDSP products add anything to the table in this specific case.