High quality pre-amp improve sound processor

It would depend on your preamp. A lot of high end preamps have a home theater pass through. Where the signal receives no attention form the preamp. And the home theater processor would still control the volume. They do this so you can have your 2 channel system be part of the home theater. Such as the amp you use in the 2 channel system and your front speakers form the 2 channel system. With out a passthrough i think you would have the 2 gain stages(preamp volume and processor volume) working against each other.
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When you build a system for both HT and 2 channel, the HT processor is usually the biggest bottleneck for getting quality 2 channel sound. Like others have said, get a good 2 channel preamp with an HT passthrough.

The sound from your processor going into the 2 channel preamp is probably not a good idea as the signal will be getting changed and manipulated an additional time from the original signal. I doubt that you can add "high end" 2 channel flavor to it. You can try it and see, and btw the effect of buffers yielding tube sound is over stated IMO, they are useful for impedance mismatches I am told.

"09-07-15: Mechans
The sound from your processor going into the 2 channel preamp is probably not a good idea as the signal will be getting changed and manipulated an additional time from the original signal. I doubt that you can add "high end" 2 channel flavor to it. You can try it and see, and btw the effect of buffers yielding tube sound is over stated IMO, they are useful for impedance mismatches I am told."

That won't happen if the 2 channel preamp has a HT bypass. Its used to deal with that exact problem.

I've had somewhat mixed results, to be critical for this type of integrated connectivity approach. I'd say that A lot would , besides preamp quality, depend on the connection used (i.e, Dirrect bypass, dedicated HT passthrough -seen on a few pieces, auxilary input, tape, etc), and the quality there of.
I've had some high end pre's that seemed to add or subtract a little to the sound, while others (direct pass-through connect?) seem to do absolutely zero to affect the signal!
Honestly, however, mostly it's seemed negligible, as we're really only talking about left and right channels of digital movie content, for the most part.
Regardless of setup, I think you'd be hard pressed (as long as gains are matched, and no other issues, noise, whatever) to have anyone tell the difference, or have problem with sound quality vs all connected through the AV pre (HT duties) , instead of AV pre's L/R out's through the 2 ch pre, when switched in! Just saying. Benefits are probably going to outweigh any potential slight sound degradation quality in a less than ideal connectivity setup, vs some pure pass-through situation.
My experience anyway. Experiences and opinions may vary, obviously