So you're saying that if you remove all of the cables from the amp and then plug the front-left cable into any input, you still get no output. In addition, if you plug any cable into the front-left input, you get no sound from the front left speaker, despite the fact that all but one of those cables works in all of the other inputs.

That sounds to me like you've got two different problems. You've got a problem with in the amp-cable-speaker chain and you've got a problem in the empeg-cable-amp chain.

Unfortunate.

The only one that could be related directly to the empeg is the empeg-cable-amp chain, but I doubt it is. It's most likely the cabling, though it's possible that some pins on the docking sled have come loose.