Can I run a Pre/Pro thru a Pre-Amp without by-pass

you don't need a unity gain at all. it just isn't imprtant like people make it out to be. just choose one spot on the preamp (like, say 9 o'clock on the dial) and set your pre/pro levels for center and surround relative to that. then, whenever you watch tv or whatever, just set your preamp to that input and volume spot. then the pre/pro will handle volume for all non-music sources.

Nope, like Lazarus said you just need to be able to set the 2ch preamp's volume control to an easily repeatable position, like 12 o'clock, each time you use the pre/pro.

No....it's not necessary, but it's VERY convenient.
What about when you need to change the volume several times during the movie?
If you're an apartment dweller, like me, fiddling with the volume becomes an art form when watching a movie late at night.