I totally agree with Dave. His suggested setup works just fine and you don't have to buy anything.
Since you are affectively not touching your 2-channel setup at all (except connecting an additional input to it), it does not affect your 2-channel enjoy at all. There is a potential for a slight degradation to your multi-channel setup, but as Dave said (paraphrase) "it's a movie, who cares". Also, it will only affect the L&R fronts anyway, so not a big deal since most of the important stuff comes out of the center channel for movies.
As long as you can tolerate the "hassle" of setting the volume control on your pre-amp to a pre-determined spot (like 12 o'clock), then you are good to go.
Enjoy,
TIC
Since you are affectively not touching your 2-channel setup at all (except connecting an additional input to it), it does not affect your 2-channel enjoy at all. There is a potential for a slight degradation to your multi-channel setup, but as Dave said (paraphrase) "it's a movie, who cares". Also, it will only affect the L&R fronts anyway, so not a big deal since most of the important stuff comes out of the center channel for movies.
As long as you can tolerate the "hassle" of setting the volume control on your pre-amp to a pre-determined spot (like 12 o'clock), then you are good to go.
Enjoy,
TIC