This is all speculation on my part but I am intrigued by the larger form-factor. The Concerto already has an ample power-supply, and puts out 10V and 0.5A max. The corresponding specs for the Jazz are 15V and 0.3A. This makes me wonder what the respective figures are for the Classic. It just might be an even more powerful beast, and satisfy those - I am not one of them - who feel the Concerto doesn't have the power to drive orthodynamics such as the LCD-2 or HE6.
Like cooper, I am keen to hear impressions of the new Jazz.
And indeed, Jan does regularly update his amplifiers. My impression is he enjoys innovation and design more than he does making a product and resting on his laurels. His regular updating is a good thing, and I think the excellence of his constant honing of amplifier performance may tend to be obscured sometimes by new FOTMs. He really is very creative imho.
Also note his recent announcement (May or June) he is cutting back his hours, workload and number of models to concentrate on new designs and on enjoying life. Well deserved, and I hope he is managing to do just that!