Hmm... I have to admit that I only thought about inputs 1-12 and inputs on the cards... but wasn't even really thinking about that the D8B has line inputs from 13-24 that do NOT have pre-amps on them, but are specifically designed for line-inputs. That's what Crash referred to as the "jackfield" on the back of the D8B.
There's "Trim" knobs present for these channels, of course... and yes - you'r passing through some sort of OP-Amp component that the trim knob will adjust. But that's not a "mic level" type of situation where you'd send the signal trough two pres in a row. These inputs are meant specifically for line-input type devices, such as the line-output of your external pre-amp. So, I have to agree with Crash, that the most straight forward way to use that pre, would be to simply connect to the line inputs of channels 13-24.
You can easily patch them to the channels you want to record to, so, it shouldn't really matter what number is printed on the input you connect to, since you can change the routing in the D8B.
...having said that, I also didn't think about the insert points, and Nuss is absolutely right about that (while considering Bruce's point about the differing signal levels inserts seem to be designed for on different devices, and that this will bring the signal in unbalanced).
IMO, if you don't have a noise issue and a signal coming in from an unbalanced input like the insert point, and the signal is strong enough for a good recording level, you're good
(I'd probably do a test recording from a line input and the insert, to see if one sounds better than the other... but then again, unless you need all these inputs simultaneously, I'd just use the line-input in the first place).
As for the sound quality... IMO, the D8B pres aren't very good. I don't know that particular Focusrite pre, but the "ISA" series has a pretty good reputation, so I'd think that it should be MUCH better than those in the D8B (...or at least have a more desirable "flavor" of sound coloration than those rather dark and unexciting sounding D8B pres).
...and generally about "fancy" pres... I have to admit that I often do NOT hear a clear difference if I record a single track with one pre, then with another pre and compare the two.
What DOES make a difference, though, is how tracks that were recorded with a "fancy" pre, "stack up." So, if I record an entire project through the same "fancy" pre, and mix the tracks, the MIX will sound different and clearly better, IMO.
If mixing a project that was recorded through budget/built-in pres, such as the ones in the D8B, the tracks don't "stack" as nicely and are missing that special "something", IMO.
What I keep hearing is responsible for that, is all that Iron in the transformers of those fancy pres... but at this point, I'm just repeating marketing talk.
All I can say, is that I like the end-results of a mixed project better, when recorded with my good pre (I only have one good pre, and that's an Aurora Audio GTQ2), than when recording through one of my many budget pres, that came built into various audio gear, or that I got free/cheap over the years (Universal Audio (Apollo), PreSonus (Quantum), M-Audio (Octane/2626/PF610), Art (Project MP), Alesis (Studio32), Mackie (analog 8-Bus), NI (Komplete Audio 6), etc. etc.... all those are just "compromises", compared to recording via a higher-end, external pre IMO).