what Elevick said is the answer - both of his posts. I'm not sure what Lexicon you're using but it's likely to be superior to your NAD - even for music, but if you prefer the NAD, then what you do is connect the the speaker outputs (ie Main L/R) from the Lex, to one of the inputs of your NAD. This can be Tape in, or any other unused input. When you watch video you pick set the volume level of the NAD to a level that is easy to remember and repeat - for example - 12:00 position. You calibrate your HT volume levels at this setting, and any time you watch HT, you return the NAD volume to this level. This way the volume control of your Lex will control your HT volumes - and pass through the NAD.
This all assumes your NAD doesn't have HT Passthru. This is the safest, lowest cost, and best sounding of all options for you.
This all assumes your NAD doesn't have HT Passthru. This is the safest, lowest cost, and best sounding of all options for you.