Kal, there is no A/D/A conversion when I play stereo mode in 2.2 setup. I run digital straight into the Trinnov, and digital out to an outboard DAC for mains, and trinnov analog out for the subs.
To integrate this with MCH you can use the Trinnov analog inputs and do A/D/A on the analog ooutputs of your processor (which is what I do).
Alternatively, if you have a processor with its own room correction, you could use this intead of the Trinnov for MCH sources. The problem is you would need an input switch for your mains and subs - one input comes from the processor in MCH mode, and the other from the Trinnov (or the outboard DAC if that is what you use) when you play in stereo mode.
Lot's of boxes, cables and expensive - much easier to do 2 channel and MCH room correction in a single processor.