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.

You would just adjust the volume on the pre/pro? Even without a HT-bypass, just set the analog preamp to a repeatable position, then adjust the levels of the L+R relative to center+rears on the pre/pro. Then anytime you need to adjust movie volumes you track at 5+ channels using the volume pot on the pre/pro, only. Not the preamp. It remains at the repeatable, fixed volume for the entire movie regardless of volume adjustments to the pre/pro.