I've been down this road many times and here is what I've observed, all in the context of the kind of system I have:

The C10/C15 digital out is really bad.

The MS600 digital out is OK, the M upsampled, apodized output is terrible. 1X output is much better. With the kind of outboard DAC you are talking about, it's going to be serviceable but not the best Sooloos has to offer. I never used it as a stand alone DAC.

The 818/ID41 digital out is the best sonic option if you want to use Sooloos with an outboard DAC. Right now I use the 818 as a preamp and Sooloos endpoint. Sonically I'd rate in as a "B-" level component. It's digital out is far superior to the MS600, again 1X sounding best. You have to fool around with M Config to force it not to upsample, so it's not as straight forward as the MS600.

IMO, if you want the best sonics, look elsewhere before you fully commit to Sooloos. Maybe this will change in the future but past history says it won't. M is focused on its whole ecosystem and has left original Sooloos owners with very little to work with...