I don't have a ton of experience with ASUS cards but I got one for a customer and it was good. I'd say to ignore the opamps for now. Unless you need 7.1 or DTS/Dolby then I'd highly suggest getting something external. There's not a lot of external products at the $60 range but that's not too far off, you could save a bit more and get a nice setup. It's been a while since I've kept up on audio products but as usual,
http://head-fi.org has good info

That said, I really enjoy my Schiit Modi for $100 bucks. USB power, small, simple circuit and it just works, no hassle. You could get one of these and a budget tube headphone amp on eBay.
Kari, if the card was defective, you should have tried a new one.