Well, I do have a "sound card" (integrated stuff on the motherboard in fact). But I only use it for plugging the headphone system directly to it. However the speaker system goes through the SqueezeBox. And the SqueezeBox doesn't use the sound card, the signal actually goes through the network instead.
Obviosly adding units in the "chain" doesn't improve the signal anything. But I am bound to have a pre-amp since the power-amp doesn't have a volume regulator.