High quality pre-amp improve sound processor

How can it not affect the sound if the signal is being passed through it? The idea is to use a preamp that is more transparent and less colored than an AV prepro/AVR.

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.
enjoy

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.