Hi Kevin,
Since you're mentioning me by name:
Yes, if you go into the D8B digitally (only), and out digitally via S/PDIF, then the D8B never converts the signal to analog as it passes through the desk (AFAIK) and, the D8B's converters will not have any impact on your sound quality.
When going through the plug-ins inside the D8B, the signal also stays digital - my point with that was, that the "quality" of the plug-ins in the D8B WILL have an impact on the sound quality... "stays digital" doesn't automatically make things sound good. So, while the plug-ins in the D8B are digital all the way through, IMO there's better software plug-ins available in modern DAWs (or DSP based, like the UAD stuff, for example).
But I think you got the point I was trying to make, and I'm using the D8B exactly like that myself:
I mostly work in a DAW, but send all my audio out of the 24 ADAT outputs on my audio interfaces, and into the 24 Tape Return channels on the D8B. I like to send individual drum channels to the D8B, which usually takes up the first 8-12 channels (depending on the project) on the Tape Returns. For the rest of the channels on the D8B, I usually "buss" signals together in the DAW, and then send stereo-busses over to the D8B (e.g. stereo busses for rhythm guitars, lead guitars, bass, keys, main vox, backing vox, etc.).
From there, I DO often use the channel dynamics on the D8B.
And for people like me, who feel very much "at home" on a "split" mixer like the D8B, all doors/signal paths are open and ready to be used, once you got the signal on the desk like that... (aux sends, busses, monitoring options, inserts, etc.) Go-Go-Gadjet-o-Octopus, if you will, haha.
Of course the use of some of that stuff WILL send the signal through the D8B's converters again (e.g. Aux Sends or analog inserts).
But IMO, converter quality is mostly an issue as conversions are "stacked". Meaning, if all the conversions in your project were done with relatively poor sounding converters, of course your project wouldn't end up sounding as awesome as it could at the end.
However, if you recorded with good converters, then theres already not as negative of an impact by "stacking" them by playing all the tracks that were recorded like that back at the same time (i.e. better converters).
If, you end up using the aux sends or other I/O on the D8B that uses the D8B's built-in converters, it won't immediately "suck" just because of that. I think in the context of a full mix, nobody would be able to tell if you used the D8Bs converters for a few Auxes or if you used high-end converters everywhere...
It's just how it all adds up... so, for example, if you add a lot of conversions from outdated, somewhat harsh and dark sounding converters together, by recorded everything through them, and then stacking 24+ tracks that were recorded like that on top of each other, PLUS multiple conversions with the same converters on the way to external gear etc.... you will most likely end up having the character of those outdated converters show through on your project.
If you are careful to record everything through higher-end, modern converters and are careful to avoid lower-end/older converters where you can, then you'll over-all end up with the character of these better/modern converters on your project. Even if you choose to run a certain percentage of the signal through lower-end converters (e.g. to use the Aux channels and external gear).
Basically, it's always the sum (mix) of things. So, avoiding the D8B converters where possible is a good idea, IMO. But you don't have to avoid them like the plague. Utilizing "some" of the D8Bs analog I/O will not immediately ruin your production. The converters aren't THAT bad