Most amps I have seen with multiple speaker outs usually have them wired parallel. It sounds to me like you are providing a 2 Ohm load to the amp instead of 4 but someone else may know more details about that specific amp. A while back I had a similar situation with a couple stereo 2x12 cabs. Each speaker was 4 ohm with a separate jack for each. Instead of rewiring the cabs I picked up a little project box and six 1/4" jacks from Radio Shack. I made two sets of two jacks wired to a third in series. This allowed me to wire each stereo cab at 4 ohms (each speaker) as a mono cab at 8 ohms without changing anything in the cab. Since the cabs had that kind of furry fabric on them, I just slapped a piece of velcro "hooks" on the bottom of the box and it just slaps onto the back of either cab.