Who says most soundcards only have one line out? USB sound devices offer some possible solutions:
Creative Sound Blaster X-Fi Surround 5.1 Pro Sound card
Sewell USB SoundBox 7.1 USB Sound Card
Another solution is taking the audio out via the optical connection into a DSP processor and then into your speakers. Or you could take your 2-channel audio into something like this...
BTW: yes the audio from you sound card can go directly into your amp