It's a sticky sort of question, though.
I think that what you're seeing is the cumulative effects of prosumer. Weakish preamps and weakish conversion meet decent (not great) tracking and PRESTO!...instant nothing!
The maintaining depth thing SEEMS (at least in my reality) to come from quality conversion and quality preamplification. When I had my converters modded (MOTU 896HD), I was stunned at how much easier it became to mix...almost immediately. All the spaces I had to place stuff felt like they were bigger and more forgiving...and since then my tracking chops have improved and I feel like my stuff is starting to sound like real recordings and not so "prosumer."
Granted, I'm probably not going to be 100% happy with the analog side of my chain until I've got all my Quad 8 built out (2 strips coming back as I type...EEEEEEEEEE *screams like an American Girl circa 1964 at the Beatles) and have Apogees or Myteks or SOMETHING like that.
At any rate...it seems to me that the MAJORITY of that stuff gets lost at the front end, and there isn't a lot to be done at the back end to regain that sense of depth because it's more than just reflective cues that get lost. The transient information that gets lost or garbled between the pres and the converters just seems to flatten things out in a way that isn't awesome. There are times where I try to embrace that as a sound...which is hard to do...but the other thing I try to keep in mind is that for the most part, as a mixer, it takes a little magic to transcend the quality of the tracks you've been handed.
Not that it isn't worth exploring...just the thing I tell myself when I'm frustrated with difficult tracks.
