Ok, try not to laugh--I used to work for Radio Smack and when they cleared out their car audio line I bought the Optimus 400W 4/3/2 amp...yes this is going back a few years.
It's set up 3-channel (dual coil sub in series and 2 6x9's) and it's not playing any bass off from the right channel, just the left. My 6x9's are still playing ok though.
If I unplug the patch cable and switch them around, it does the same thing, no bass off the right channel.
If I set it up in 4 channel and run one sub coil off the left and the other off the right, it's ok. Does it sound like the amp may be messed up, or is there some trickery to this?

**edit: Nevermind, figured it out myself. The switch on the amp doesn't sum the two inputs, just uses the left input and sums the outputs (places collective 'dunce' hat on head)