The Diablo is more of a hot rodded classic tube amp... I've never played one but I've read up on them. High quality amps... Good for a lot of different styles from Blues to Metal... These amps are closer to the quality and performance of a Mesa Boogie than any mass produced brand (Marshall, Peavy, Fender...etc.)
The XXX is about the same way... Versatile amp. Mainly built for hard rock or metal... Still gets great blues tone with the right guitar and effects. Great quality and a well known amp...
The JSX... Well... it's obviously an amp built for modern tones. Not going to give you a great blues tone right out of the box but it will cover just about every other tone very well... Even with the ability to switch from solid state circuitry to tube circuitry... giving it that much more versatility. Most people wouldn't want to switch from tube to solid state... But if you're a metal guitarist and you find your tone getting saturated too much and you can't hear all your high notes or they lack definition... that's when you'd switch circuitry without having to add a pedal to bring out the highs or tweak your treble all the time..... So it's a modern amp built for modern music.
Personally... I've been dreaming of a Genz Benz... I own a TubeWorks (Genz Benz bought it and the amps that they make are basically the same as the TubeWorks ones...) it compares pretty closely to the tone of a Marshall JCM or a Mesa Mark IV. I would personally get a Genz Benz out of those three choices. They're all killer amps though. Hope my insight helps a little.