They may have lightened up the chassis. I know Marantz made the mistake of going to a smaller, cheaper chassis with the 8400, something they corrected with the x500 line. I would not use the total weight of the unit alone to determine any kind of qualty.
Not sure what to think about THX cert these days. The 1014/1015 are select/select2, but I know from experience with the 1014 that it doesn't really live up to it's rated power. Kenwood also had a rather low priced model that was THX Select cert too.