It sounds as though you have somewhat isolated your problem, if I am understanding correctly.

You can take an RCA cable from the empeg, let's say right rear, which you know works, and plug it into the front left input of the amp and no sound is produced from that speaker.

If so, then it is a problem between the amp and the speaker. Just to make sure you do not have multiple problems, plug the front left RCA from the empeg into the right rear of the amp and make sure that it produces sound. Eliminate the empeg or cables from the empeg to the amp first.

It sounds like this is where you are. If so then you are on the right track. Check the easiest things first, whichever those are. Wire connections to the speakers, wire connections at the amp for the speakers. Possibly the wire itself is pinched and broken somewhere, and worst of all, possibly something is wrong with the amp.