The MS600 digital out is OK, the M upsampled, apodized output is terrible. 1X output is much better.
Your statement might have been a lot more credible if you had simply stated that the "Apodized output wasn't to my liking and I preferred the 1x output" or similar... using the word "terrible" just exhibits your long stated bias against Meridian, as any reasonable reader will understand that components at this level of the food chain cannot be reasonably classified as "terrible"... "not to your taste" perhaps... but "terrible" is simply not credible to most reasonable people.
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.
It's quite interesting that you find the Meridian 818 to offer the best Sooloos End Point performance... especially when you appear to dislike Meridian's Upsampling and Apodizing so much.
All Meridian 800 series components Resolution Enhance as well as Upsample / Apodize ALL Inputs to a 'High-Rate' 24/88 or 24/96 Stream internally within the 800 series component. All internal Processing is then carried out on the High Rate stream... with the DAC's always being fed by the High Rate Upsampled & Apodized stream.
This occurs with ALL Inputs which are fed into the 800 series component... including the input via the Sooloos ID41 card.
All the above happens by design, with the User having no option to change this. The setting within the MConfig ONLY allows the User to force the Digital Outputs to Downsample the Upsampled / Apodized High Rate stream back down to a Standard Rate stream... in order to feed components which do not accept high rate inputs.
So your own "blind testing", where you thought you were disabling Upsampling & Apodizing, seems to confirm that you may indeed prefer Meridian's Apodized or "Pre-Ringing removed" output after all.
As I say, an interesting result from a real "blind test"!!!