Need help with pre-out/main-in question

I'm a little confused on the pre-out/main-in thing... I bought a used stereo receiver (NAD 7400) with pre-out/main-in jacks and the factory jumpers are in place. I'm thinking of creating an old-school but high quality 5.1 system by adding a digital surround processor like a used Sony EP9ES and outboard 3-channel amp to make a 5.1 system. Will it be ok to remove the jumpers but leave the "pre-out" jacks open and only use the "main-in' jacks to connect the front L/R output from the processor? Since there's no digital inputs on the NAD, why would you need to pre-out anything? The Sony processor has it's own volume control, as well.

Either I'm missing something or the owner's manual isn't clear enough on this feature. Essentially, I just want to use the power amp part of the NAD while in home theater mode and connect all digital sources to the processor but still use the NAD's pre-amp and volume control for my stereo sources. The diagram in the NAD's owner's manual are from the days of Pro-Logic and it shows the (analog?) processor being connected in a loop to both sets of terminals much like an equalizer would be. That makes sense if you connect a HiFi VCR to the NAD and then need to send the analog output for processing, but what about with digital?

Please advise..