For all non-KDFX units (all K2000s and K2500s without KDFX) it works like this:

A Outputs:

Any Layer who's OUTPUT Pair (in OUTPUT Page) is set to A (FX) will first go to the A Output Jacks. If there are no cables plugged into the A Output jacks the signal will go through the standard FX and then to the MIX Outs. If there are cables in the A Output Jacks the signal will not go to the FX and thus will not go to the MIX Outs.

B, C, and D Outputs work the same way, but differently from the A Outputs as follows:

Any Layer who's OUTPUT Pair (in OUTPUT Page) is set to B, C, or D (Dry) will first go to the B, C, or D Output Jacks. If there are no cables plugged into the corresponding Output jacks the signal will go directly to the MIX Outs (B, C, or D never get FX). If there are cables in the corresponding output Jacks the signal will not go to the MIX Outs.

The key thing to remember is that there is no way to send a signal directly to the MIX Outs. MIX gets whatever is not going directly out of the A/B/C/D Outs. So, if you've got 2 conductor cables in A+B+C+D, there's nothing left to go to the MIX Outs. And the other thing is that only Layers sent to A (FX) will get FX (via the MIX Outs), and then only if the signal isn't being hijacked at the A Outputs.