I think htat IF it is possible, it's going to be a piece made by Eclipse for that purpose. That's how it is with the Sony anyway. A good stereo shop selling Eclipse components should be able to tell you if they have done something like this before. Don't tell them you have an Empeg, or you'll get funky answers. Just tell them you want to hook "Another stereo" into those inputs.

Also, if the sub control is of importance, do you have other options there? My Rockford Fosgate subs have a little device that connects to the amp by a phone cord, and is basically a remote volume control of sorts just for that amp. Many other companies can do this, possibly yours can too?