Differences between Martin Logan subwoofer lines

What are the essential differences between the 3 subwoofers that make up Martin Logan's Descent, Depth & Grotto and the newer Descent i, Depth i, and Grotto i? Are the newer ones worth the extra money? And there doesn't seem to be any mention of equalization with these woofers; any concern?
Not saying that members on this site cant offer input, but maybe you would have good results over here :


Isnt Google great ?!!!
I would say woofer size and frequency response