I asked mattTCG about the extension in particular, and according to him, the LCD-2 should be more along the lines of a Denon, in which it's very strong down to the upper 20hz region, around 30hz. HE-400 starts a gradual decline at 40hz, and its 30hz isn't as strong in comparison, if I'm getting that right. Because there is less of it quantity wise, I assume it's more accurate. HE-400 is a bit bass enhanced as it stands. Square waves of LCD-2's bass show that it is very tight down low, thus will provide more authority there.
A lot of people have said LCD-2s bass is more impactful despite having less quantity, including Purrin. I know it may be witchcraft to MadLustEnvy, but that's just the popular consensus on the LCD's bass. HE-500 for instance, also has more bass quantity than LCD2 according to others.
Rumble I can't comment on. I vaguely know what people mean by it, but it's a word that I rather not describe with music, as it's rather ambiguous.
This is pretty accurate on the bass comparison between the lcd2 and he400. But, the lcd3 is on an entirely different planet. I wouldn't even begin to make comparisons with it and the he400...it's just that much better.