Hmm weird, well that would have stopped it from only using a certain amount of speakers anyway, or atleast it should have done. Make sure it hasn't switched itself back, like maybe you didn't press Apply, oh and check on the back of the speaker that it is set to its real setting and not on fake surround.
Now, it could also be something else. Go back to the Control Panel and onto Sounds and Audio Devices like before, and instead of going to Advanced, go into Volume. Set all the amounts to the same as the other speakers and check that some are not muted or down low.
You might find that this might be the case since when you said you switched to fake surround, all work, and this would be because it would be working from a stereo system, and not from the individual sound channels so that might be why
I hope this helps