How do I connect my emotiva mc700 surround sound proc to preamp

Whats the difference between using THRU and normal?

If the RCA3 is in normal mode, the preamp volume control still active.
When RCA3 in signal thru mode, the preamp volume control will be disabled, you will using the emotiva mc700 to control volume.

So.if I don’t use the THRU option then I can use the preamps volume control as the master volume control ?

No, it only control the front channels volume, the rest of channels still need to use the emotiva mc700 to control volume.
Using the pass thru option and let the emotiva mc700 volume control as the master volume control is the best way to setup home theater system. 

Wouldn't it be simpler to have just one volume control vs 2..

Sorry, I just revised my response.

I have only a 2 channel setup.. left and right so in that case it wouldn't matter right ?

If that is the case, you can connect the emotiva mc700 LF and RF output channels to emotiva mc700 ANY of the three RCA inputs, set the emotiva mc700 volume to maximum, or to the level matching other sources, and use the esoteric c03 volume control as master volume control.