Need help getting my subwoofer to play from my computer

Firstly, active subs for home theatre's have a built-in preamp, but they're still meant to be used with in-conjunction with a proper amplifier. I would recommend buying an AV receiver since that will solve all of your problems in one and give you room to expand your audio system and computer, without depending on what your motherboard supports. Take a look at your local hifi store and see if you can get a demo of a few. Personally I've had the best experience with Yamaha AV receivers and currently have a Yamaha RXA2050 connected to my PC.

Although to fix your current issue, it sounds like your sub is setup incorrectly. Since you're not using a receiver, I would use both the Left and Right RCA inputs at the back of your subwoofer instead of just one or the LFE input. Using both RCA inputs at the back of the sub puts it into 'stereo' mode which forces the sub to accept ANY signal it's given. The sub then uses it's own crossover frequency (configurable using the crossover dial at the back) to filter the rest of the audio out.

The LFE input is it's own channel, infact it's what gives you the .1 of surround sound. Just like there are certain sounds that are made to be played in the surround channels in a movie, there are specific sounds that are meant to be played through the subwoofer in a movie. For this to happen, the sub needs to be connected to a receiver which will work out which sounds need to be played through the subwoofer. As such, the crossover frequency at the back of the sub is disabled as it's no longer in control, all of the control is done at the receiver.

So with that said, it sounds like when you do a sound test through the Realtek control panel, it's testing the LFE channel. But when you play music, that's only stereo which contains a left and a right channel and doesn't have an LFE channel which is why you don't hear anything coming out of the subwoofer when you play music.

Also, debunking the centre speaker issue is simple. Your motherboard's CS-Out port is used for both the centre speaker and the subwoofer. So if you had a centre speaker, you would plug it into the same port on the motherboard as the subwoofer. This is why when you turn the centre channel on, you get audio through your subwoofer - i.e. your subwoofer is trying to play audio from the centre channel. In most movies, about 60% of the audio track is playing through the centre channel, so it comes as no surprise that there's a reduction in audio range from the front speakers when your turn the centre channel on since it's trying to be the centre of attention