Well, you CAN connect to the amp via analog if you wish. Its an unconventional setup though. Make sure that the amp doesn't convert an incoming analog signal back to digital for additional audio processing though (a few do).
You can also use SPDIF directly from the ST, but are bound to Dolby/DTS formats, which I tend to not recommend due to their lossy nature.
If the SPDIF is lossy, can I connect H6 direct to speakers? Well, the website say that xonar H6 is 7.1, then what is it designed for? Can H6 work without AMP?