Okay this problem is really weird, but basically the speakers seem to be out of phase when the + of the amp is plugged into the + of the speakers and so on. The sounds seems to be coming a bit from behind me on the left side. However, when I turn on the Chorus the problem seems to go away. And so I payed a visit to Marshall here, and they said the amp was fine.. wtf?
If I had to guess I would say the chorus is adding a frequency that balances it out for your ears but that there is a circuit or component in there somewhere that went nutty. I think that might be hard to diagnose and fix. I doubt it is the speaker or power section but I could be wrong. Good luck.