Does MC recognize your onboard soundcard? If so, you can set MC up to reroute bass to the sub channel.
Should not be a problem. Just split off the SW feed from the Center/Sub output and send it to the amp.
MC will do it if it recognizes the soundcard and the soundcard is set up for 2.1 or whatever speaker layout you have.
Yep. For a 3.5mm soundcard out, just get a 3.5mm to RCA splitter and figure out which RCA is the sub channel (it *should* be R).
I will say, onboard audio isn't great, and has a much higher likelihood of introducing bad EMI / background noise depending on your speakers / amplification setup.
Definitely consider an external D/A solution when your budget can accommodate it.
Also, how crappy is this subwoofer? It should be able to handle a bridged amplifier. Just make sure you run a low pass on it and figure out a comfortable place for the volume on the amplifier relative to the line level signal coming out of your PC. In this situation, you're better to have the amplifier louder, and attenuate the subwoofer channel digitally, most likely. My subwoofer uses a bridged 200w NAD amp as whatever plate amp it had was dead. A DSP low pass is all you need to turn it into a 'sub' amp