It sounds like you have the wires hooked up correctly. Make sure the mini-phono plugs are plugged in all the way and make sure you use the correct outputs of you sound card are hooked to the correct inputs on the SWS, the colors are not the same for every card. (I think the orange = center/sub, green = front, black = surround) Just wondering which Promedia system do you have 2.1, 4.1, 5.1?
With regard to the volume control on the sound card or (computer controls) I max out everything except the main volume control (listed as "play control" with Windows XP). Don't use the treble or bass controls at all, set it for flat EQ. If you are using a Soundblaster card with EAX make sure all EAX effects are turned off and select the correct speaker settings for your setup (if you have 4.1 setup, select 4.1, etc.) If you have a newer Soundblaster card, such as Audigy 2. Under the EAX Advanced HD console, CMSS 3D tab you can select to have the all the speakers play music or just the front ones. You can choose from three different effects: CMSS 1, CMSS 2 and stereo surround. Note: CMSS effects is different than the EAX effects. If you want a little more bass select the stereo surround, but you can easily experiment with the other ones.
I have the v2.400 (I think that is what it is called with out looking it up) upgraded with the preamp of the 4.1s'. I have the subwoofer knob at about 11 o'clock or where the little white line is (you know which white line) The main volume knob is at about 9 0'clock. The volume control for the SWS is never turned past 9 o'clock. I adjust the volume using the computers volume controls (my keyboard has volume up and down), instead of having to adjust two separate volume controls. My SWS is set with 60HZ low-pass filter and 180 phase. With regards to sound quality a lot has to do with the room acoustics, having the SWS in a corner seems to work the best. I have noticed that at least with the v2.400 setup the main sub is more midbass (punchy bass) with very little low end bass and the SWS is all low end (boom/room shaking bass) with little midbass (obviously with the low-pass filter). Hopefully, this will help you.