Thanks all, so far. I'm sure the sound card isn't optimal, but pro's used them for years with good results. I've had a usb m-audio, and the sound wasn't even close, even if it was cleaner. Unfortunately, I need a mixer, as I have all kinds of inputs to deal with. I would like to get a nicer mixer though, possibly usb/firewire, as you say.
My old polks weren't nearly as harsh though, so I kind of think the speakers might be the biggest contributor. I've never heard the spendors, so I'll try to look for a chance to listen to them. Are there others that are especially good that way? I'll remember the soft-dome tweeter part. I hadn't thought of it, but it does seem like metallic tweeters could be more harsh.
As far as itunes, I only import apple lossless. The eq in itunes is a good idea. I tried it on the treble reducer setting, and it is much better. At one time, I stopped using it for some reason (bad distortion, i think?), but I don't hear anything odd, so that might be an option for at least a while. Anyway, thanks again, kj.