Not to put too fine a point on it mate but you seem to have got something of a dogs breakfast there.
A gamers sound card. Meant to work with headphones and computer speakers. A domestic hi-fi/HT sub. Meant to work with an AVR or receiver. Plus a set of pro studio monitor. Meant to work with an audio interface.
Hold up on flashing the cash for a bit. Work out what you want to do first. Or you risk making things worse.
You can get the whole thing working ok for very little. I think what the guys in your situation do is buy a line splitter. Attach that to the line outs on the soundcard. Send one pair to the sub and the other to the mains. I'd use the stereo link option on the ADAMs. That way you retain a individual volume control for both mains and sub. Balanced in out of the question so don't bother with XLR.
If you are thinking of getting an alternative sub consider the EVE TS range. Same price as the Monitor Audio and the Sub7 but crucially for the home user it has a system level volume control as well as the usual sub level, variable low pass, fixed high pass and phase controls. It also has LFE (or party) mode where it operates up to 300Hz for a right banging sound. All controls are available on the included remore too.
Check it out. Same price, better features. 8" model is much superior if you can afford it and have the space.
http://www.eve-audio.com/index.php?page=TS107
Then take a long look at the soundcard. Start thinking about getting something more appropriate and versatile when you can. Focusrite 2i2 would be an improvement for not much more money but better to spend ~$200-$250 on something from MOTU, native instruments or something further up the Focusrite range.
That's from the viewpoint of going fully down the pro audio route. In keeping with your best product. Your mains. people with more gaming or HT experience will probably recomend something different.
