Have you checked out EMG TV? They have demos of Bobby Vega using those EMG JX's, the JVX, and the old J set. There is another vid on Bassplayer TV. Those should give you an idea of how those pu's differ in sound.
I made the change to EMG's just a few months ago.
I made a quick sound clip on my Jazz bass with the older model EMG J's (no onboard preamp though):
http://www.youtube.com/watch?v=BmpxAneYw5Y Though I've watched those vids, I don't really know how big a difference the old J set has with the new Jx's (which has more headroom, they say). It would be interesting to hear them on the same bass.
If you're coming from a passive set up, the sound difference is night and day. I just changed from passive Fralin single coils to these EMGs. I like both sounds but the EMG's are much more versatile. But then again, since you're using the Dimarzio ultajazz set, you're probably used to a more "hyped" up tone compared to passive single coils.
