I can answer some of your questions:
Pre means the signal coming out of the DI is just what's going into the input (after going through a tube buffer stage) and through the effects loop (ie. effects but no onboard EQ). If you select post, you get not only the onboard EQ, but also the volume control.
When I had an F1X, I liked this arrangement a lot, but I didn't like the fact that the effects loop was before the EQ, which is good for most tone shaping and filter effect but not so good for reverb and delay. So I modded mine with a post EQ effects loop and put those effects in that loop. I then had to take the post out from different spot so that it would include those effects. It worked out pretty well! Benson now has that unit.
HTH,
Edwin
PS I think it is possible to add a master volume to a Series I. I'm sure Mica would be happy to help you out on that.