Understanding the Sooloos Architecture is important, but in summary
** The 808.x will Replay a 16/44 file from its "Internals".....Upsample, RE & Apodize to 24/88......Encrypt it with MHR and pass it along to the next Digital Device
Both the C15 and MS600 will do exactly the same thing.........play the file from its "Internal Components".........Upsample, RE & Apodize to 24/88..........also Encrypt to MHR if required
After that, it will be down to the Design & Layout of the MS600 versus the Control 15.........the Control 15 is an evolutionary product from the independent Sooloos days, but Meridian have made significant changes internally that many are happy with........by contrast the MS600 is a ground up design by Meridian and likely to offer Low Output Jitter
There aren't enough MS600 Users here to report and I think the only way forward is a Demo.......hence my suggestion above of a long term home demo..........that way, you can judge for yourself
Having said that, I think the ID40 Card within an 808.3 (or possibly 861v6) will still offer the Ultimate Level Performance from a Sooloos system..........but there is obviously a very high price to pay for that last x%