Hi. I'm wondering how to get a stereo tube amplifier to work with a powered subwoofer. There's a couple of tube amplifiers I'm interested in, but I don't know how to get my subwoofer working with it. The basic setup is this: PC sound card to amplifier (to subwoofer how?). Currently I use a reciever to do all of this, but I'd really like to switch to a tube setup. Thanks.