That is not really how it works.

Line-in inputs only carry sound. These are like the ones you would find on an amp or such device. The outputs from another stereo would be connected to these inputs, and they simply carry the sound over through the Empeg to the amp that it is connected to.

When it is hooked up right, the Empeg will work just as it always has done. But now, you can go to 'source' in the menu, and then 'Aux' and you should now be able to listen to the Radio/CD Player/Whatever that you have plugged in. If your other radio shows RDS information, it will be displaying that. The Empeg will not be displaying anything all that useful (you could turn the visuals on or something).

Hope that makes a little bit of sense now. This kind of setup would look a bit like this.