High quality pre-amp improve sound processor

Another thing you can do with this type of setup, is connect 1 source to both preamps. For example, if you have a universal disc player, you can run an IC from the front analog outputs to an input on your 2 channel preamp. You can also run a digital cable to your HT processor for movies.

So, just to clarify what has been said above in case it's not clear, rather than have your stereo source go into your prepro and then to the stereo pre it would be the reverse. Your stereo signal would go directly to your stereo pre, and the front L/R outputs from your surround source would go to a HT bypass or other unused input on your stereo pre and then output to the front L/R inputs on your prepro. This way, for stereo your prepro is completely out of the system and to go to surround you just change the input on the stereo pre (if no HT bypass you just need to set a reference volume level so your fronts are at the proper level for multichannel duty). Installing a stereo pre after the prepro, as mentioned above, basically defeats the purpose. Hope this helps and best of luck.

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

"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.

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.

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.

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

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.