Ok I have watched parts of five movies and listened to music for about three hours. These observations are subjective but since the cube is working under the same setup as full marty and in virtually the same location, at the same volume level (-5 from reference for movies, -15 to -20 for music). I am going to use a little of the flowery language so don't beat me up. For music I really prefer the cube. It seems to handle difficult passages of music with tighter smoother bass. For example Black Eyed Peas Missing you is a song with lots of different octaves mixing together. While I was listening to that song I ended up toe tapping and closing my eyes and just grooving to the beat. I don't do that very often. Midbass is definitely punchier with the kick drum being felt more in your chest.
The dampening I used for both was the same amount, plywood the same, both heavily braced. For movies gunshots, punches, swords slamming into walls is better with the cube. Their is the drop off of the lowest bass which I can hear as well.
20hz to about 28hz is definitely better with full marty. I think the second cube with the uxl18 driver will take care of that weakness. My room is 3000sqft in size. overall I am very happy. When my second cube is hooked up i will give you more impressions.
On a side note I really enjoyed our build night with Jbrown and Warren. It was cool to talk speaker stuff with someone who really cares about it. Friends and the wife just nod their head when I am telling them about my system. Going to get about 5 sheets of mdf on monday.