Need Preamp Advice Please

Not a problem. All really need to do is get a 2 channel preamp, and hook things up like a 2 channel system normally would be. Then you can simply "loop" your receivers "main left/right" outputs into an "auzilary" or "tape loop" input on your 2 channel tube preamp, and simply engage that input on your 2 channel pre when you want to watch movies!...and of course the other channels(?) would be connected like normal to your receiver.
Now some of the tube preamps actually have a "processor loop",(AR LS16, Sonic Frontiers Line 1, etc) which functions just like a tape loop virutally. It's a simple "through-put" for processors, EQ's, etc.
Whith these set-up's, you can simply keep your receiver off while doing 2 channel, and keep the intergrity of both systems intact and separated!
good luck.

It would be better to use a preamp with the processor loop to avoid having two volume controls in line with each other.

There is an EAR 834L for sale from a dealer for 1100 . It is hard to beat for your application. It can be setup as described by Lthkeepr or the dealer can even help. I have no affliation with them but in my philosophy when you can get a great deal through a dealer it feels like a bit more protection for you the buyer.

The ANthem line also has surround processor loops.