Maybe?
Off the top of my head, it sounds like like the card is not seating properly, or fully. Ha ve you checked the "golden fingers" on its base to be sure they are are clean and intact? Or, perhaps it could be the PCI slot at fault. Have you tried it in a different slot?
The only reason I can think of that your PC might restart on removing the speaker cables is that it wiggled the card loose. Do you have it secured in place with a screw where the outputs are?
I'm no expert, but I have the XFi Xtreme Music (basically the same except for the front panel), and have had no such problems. There is always the possibility that the card is defective- can you try it in another system?
Not much help, but some ideas. Good luck!:grinthumb