I think both teedge77 and I were confused by the same message - you seemed to be saying that you only had the optical out from your amp connected, but I now assume you were talking about the optical out from your motherboard; and then you said your amp doesn't have surround sound at all - again, I am assuming from your later post that your amp is capable of surround sound but isn't receiving a surround signal from your motherboard. With these assumptions in place, I think I know understand - but unfortunately it doesn't really help me help you fix the problem!
Either way, the SPDIF out is only capable of digital transmission of the audio signal, any analogue signal would be rejected and "gated" by the circuitry. So for whatever reason, your motherboard/LMCE is only directing your (digital) audio signal to the SPDIF connector when you choose the Analogue setting (bizarre!) and no signal at all when you choose the the SPDIF signal - this is the only set of circumstances I can think of that would explain the behaviour you are seeing.
The only thing I can think of that would allow for this combination of weirdness would be a driver problem, or very remotely a hardware problem on the board. Either way I don't know enough about the drivers to help any further here - over to you teedge77!