I think mixsit and I both gave the same answer to that question in slightly different ways.
The ideal in gain staging is to get all the gain you're going to need as soon as possible and the try not to turn it back down and definitely don't turn it down just to turn it back up again. In most cases that's going to mean setting the external pre so that it's sending a signal right around 0VU (+4dbu) and then leave the interface right at unity.
But if you're trying to actually push these pres into distortion (which may have been an assumption on my part, but is at least half the fun), then you will at some point have to turn it up quite a ways past what we'd consider a reasonable line level. Then you will have to turn it down some to get that line level at the converter.
On most pres, that's going to mean turning the pre/input/whatever knob (usually the one furthest left
) until you get the distortion you want and then turning the post/output/thing (usually over toward the right) down until the interface is cool at unity. Some don't quite work that way and then you do what you can.
If you're going through the insert, I'd try to get most of the necessary gain from the interface, but if it starts to distort before the pre, then you'll maybe turn it back a bit and then get the rest of the gain at the pre. Then adjust the output of the pre to bring it back down to nominal.
But like I said, all of this is generalized theory, and you can't know exactly how one piece of gear is going to interact with another until you do it and hear it. Plug it in, follow the above instructions, if it doesn't sound the way you want, turn the knobs until it does. Nobody's going to come in and tell you you're wrong. If you don't have
unwanted distortion or
intolerable noise, you're good.