I'm not familiar with the AKG K18 headphones, but unless they have specific resistors (as do some very high end 'phones), you won't be able to run them from a T-amp, which is designed for speaker output, not standard headphone output. But to answer your main question, I would say that the soundcard is important, and that the primary job of a good amp is to bring out the pure, high quality signal it is receiving, not to color it (desirable tube affects aside); so assuming that the card is unimportant because "the amp alters the sound signal anyway" really is quite an error. I do not recommend an audigy card, since a cheaper and, as consensus would have it, better sounding option is available in the form of the Chaintech AV-710, which I use myself. I have compared it to the hollow, bland sound created by my previous generic card, and found it vastly superior.