I think that when you run the large in stereo, the drivers are 8 ohm. But when running them in mono (in series I think...) then it's a 4 ohm load. I'm not 100% positive about that, but I think that's why you're getting the results you're getting.
And if you run one of the drivers 180 degrees from the other you'll get no bass as they'll cancel each other out. I've tried that with my subs once to see what'd happen, and they just cancelled each other out. Not terribly helpful like that...
