From what I've read, the only difference between the 7600 and 7500 is the addition of an EQ. The WK3700 has an EQ AND better speakers. I still don't see what the big deal about the WK7500.
I read that the 7600 also has a few more voices and rhythms and smoothed out the performance of the draw bars so you don't hear the "click" when you use them in real time.