hello!
As i recall...
and it was AGES ago now, i got a rough level matching the two preamp gains, then for fine tuning, sent a test tone into the pres and monitored the levels with Rn Inspector so they matched.
The Idea came from the manual for the TG2 where it mentions doing something of the sorts, i believe it also mentions trying everything for a different sound, including running through the D.I input as the impedence is different.
I did do this for *ahem* mastering for a bit, but im mostly using Ozone these days, so haven't done it in a while.