Ive heard the Abyss & ive heard the M&K's larger 12" version i think it was the MX75 (dont remember model) Both of those subs are good clean musical subs.
The MX70 will bkend better with your Mirages since its dual 8".
The Abyss will play lower being a 12" but may not blend quite as well as the M&K.
Since your in a dorm you probably cant pump up the volume too high im assuming. So for $50 you cant go wrong with the MX70. M&K subs are known for being tight, fast & loud. So the push/pull designed dual 8" will play nice. The sealed 12" is a good option but the $150 difference IMHO is not justified in your case.
One more question, what type of music do you like?
The Abyss will be better for rap, hiphop, electronica since it digs deeper. But most music doesnt get much lower than 30-40hz which makes the MX70 still a good choice. I like a good 8" sub myself, very clean. But if high SPL in a large area then 8" just dont cut it for me.