Believe it or not, I typed something even longer than the below, but kinda got stuck, so I decided to start over, trying to make it short. This is the short version, hahaha
Here are some opinions on different use-cases for the D8B:
A. Tracking bands WITHOUT a separate DAW-computer
A D8B together with an HDR/MDR are a KILLER combination to record bands/artists. It acts like a huge harddisk-recorder with built-in effects and gives tons of options to tie in external (analog) processing gear. If you have your D8B and HDR/MDR connected DIGITALLY, it's even better, since there's no quality loss between them in that case. Otherwise, when connecting the multi-track recorder via analog connections, these conversions happen every time you press record:
1. Analog source converted to digital as it enters the D8B
2. Digital signal converted to analog as it exists the D8B and goes toward the HDR
3. Analog signal converted to digital as it enters the HDR
4. Digital signal converted to analog as it exits the HDR and goes toward the D8B
5. Analog signal converted to digital as it enters the D8B
6. Digital signal converted to analog as it exists the D8B on the way to the speakers (this last conversion plays no role, when sending via S/PDIF to a Stereo Master-Recorder. The conversion is part of what you hear over speakers connected to the D8B, though)
If using the D8B together with a digital multi-track recorder for the purpose of tracking bands on up to 24 tracks, then it's best to have the recorder connected DIGITALLY via ADAT, TDIF or AES. Which digital card doesn't really matter, if both ends have the same type of digital connections. The goal is to have the signal remain unchanged when transferred between these devices.
B. Tracking bands WITH separate DAW-computer
This "can" work just as well and even better than scenario A above, but here, too, it's better if the connection between the audio interface and D8B are all digital/lossless.
However, it often times makes a lot more sense to record directly into the audio interface instead of going through the D8B, though, since modern converters are usually better, and the Pre-amps on the D8B also don't really hold up to what even many budget interfaces can do nowadays. So, this sort of setup has a limited use, IMO.
Besides, I think this really only starts making sense when being able to transfer (close to) the full number of Tape Outs/Returns from the D8B to the DAW computer and back (i.e. 24 out, 24 return). IMO, the usefulness of doing this for only 8 tracks or so, is doubtful, and the added complexity for a questionable benefit is often not worth it, IMO.
I could write a lot of pros/cons in this section (and did... but deleted, since it was getting too much), but what it comes down to is that it's often a lot easier to use a control surface with the DAW if all you really want is to mix with "real faders" and go straight into the audio interface with your audio, instead of dealing with all kinds of possible hurdles and possible sound degradation (depending on connections) when tying in an old digital mixer, etc.
And you can use the D8B as a control surface, too! Via the ProBox (or with only 8 faders via the built-in HUI mode)
C. Using D8B as audio-switchboard / routing central
This pretty much always makes sense, if you have plenty of signal sources and destinations, and need a way to patch them in useful ways, without having to physically change the cable connections. For this, it makes the most sense to have all analog cards in your D8B, since you'll get to tie in a lot of analog processing gear this way. You'll "only" get 2 conversions when using external ANALOG peripherals this way - on the way out of the D8B, and back into the D8B. So, this is not nearly as bad, as when connecting a digital recorder to it by analog means.
If you have DIGITAL external processing gear (like your Lexicon, but with AES/EBU connections), then it could of course also make sense to have an AES expansion card in the D8B to tie in the digital processing gear without conversions. But this comes at the cost of the flexibility to quickly connect something else to these I/O ports... analog is always the lowest common denominator.
D. Same as point C, but with better converters
You could do exactly the same thing as what I'm describing in point C, by having all digital expansion cards in your D8B, but adding better external converter boxes that connect to the D8B digitally. The guy I bought my first two D8Bs from (at the same time), used a couple of Lynx Aurora converters with it, for example. This is a way to "upgrade" the D8B, keep it useful and at the same time make it sound better... but is usually cost prohibitive. Who'd buy thousands of dollars worth of converters, to make a 20 year old mixer sound better, that can be had for a few hundred bucks nowadays. Just saying, that this would be one way to connect a lot of external gear while having modern and higher-end audio and converter quality. This still doesn't improve the quality of anything you connect to the analog inputs or aux sends/analog buss outs etc. on the D8B, though.
Anyway... "point D" is probably not very practical, unless you work for a company that makes converter boxes and have free access to them
E. A combination of the above
Something that gives you access to most of the benefits of the different ways you can use a D8B that I'm describing above, is to simply put ALL of the D8B's analog inputs and outputs on a patchbay. (And the analog I/O of your audio interface and outboard gear as well.) That's how I'm using it myself.
I have 24 DIGITAL channels of I/O connected between the D8B and my DAW computer (via ADAT in the Tape I/O slots), but all the (analog) Channel Inputs, Inserts, Aux-Sends and Buss-outs go to an (analog) TRS patchbay. Even the D8B's XLR inputs go to a separate XLR patchbay/stagebox, to make them easily accessible right next to my other pres/audio-interface's pres (that I also put on the patchbay).
I don't have nearly as much outboard gear as Crash (I really wish I did, though - amazing stuff!! Impressive!!!), but even for the stuff I do have, I don't usually use it all at the same time. And that's where the patchbay comes in. A lot of it is "normaled", so some of the outboard gear I do have is by default connected to the D8B's Aux-Sends etc. But with a patch-bay, a signal flow change is always just a few patch cables away
So, if you put everything on a patchbay, you can quickly re-configure your setup to allow you to track bands, or work on your Synth Magnum-Opus, using all the external processing you can get your hands on
...and for ADAT-routing, I still get a number of options via the control panels for my Universal Audio Apollo 8 and PreSonus Quantum interfaces (...well... via the DAW for the PreSonus interface).
What I really wished I had, was a much larger digital patchbay, though, so I could connect all the ADAT I/O to that, and re-patch any time I want. But with the rest of the setup, I can usually work around this... if I win the lottery, I'll get a huge digital patchbay with lots of ADAT I/O on it, though
Sorry for publishing a second "novel" in a single day. I just thought a breakdown like this, for different ways to use the D8B and the downsides of some ways to connect it, could be useful for some others who are reading along. I hope I'm not stepping on anyone's toes, who has things set up in a way that I describe as less-than-ideal here. In the end, it's all in the eye (or ear) of the beholder...