As an owner of both the Sapphire III and Sapphire XL I can tell you with confidence that the Sapphire XL just takes it to a new level. Smoother, more natural, more transparent, lower, deeper, richer bass, more detail -- a considerable improvement across the board. The Sapphire XL is a remarkable speaker, especially at its price point.
The sonic signatures are different, though, IMO. The Sapphire III can be a little "hot" up top and a bit forward. The XL, never. It's very smooth and easy to listen to, without sacrificing openness and detail, with a laid back presentation.
HTH.