REL Subwoofers: Version 3 vs. Version 2

Does anyone know what the differences are between the mk2 and mk3 REL subwoofer series are? I have the opportunity to buy a used Stentor 2, and would like to know how it differs from the 3 series. Also, is the Stentor too big for a medium sized room (16x20)? Sumiko suggested that it might be, and that I might be better off with a stadium 3 or storm 3. Advise appreciated! Thanks.