First, if you're happy with the JC-1s, I'd stay with them.
Second, if you really must try the Pass amps, the X350.5 is supposed to be more liquid and pleasing, as well as the more current model. While the X-600s are arguably the best of the X series and really do sound great (I have a pair), it seems like you don't need the extra power.
Third, the only real way to make a determination of which of the three amps is best would be to audition them in your system. It would be real hard for me to be able to discern a difference between these three great amps without knowing how all the system synergies will work out. Is there any way you could try the Pass amps at no cost or minimal cost? (Of course, you could always buy them used then sell them for about the same price.)
Second, if you really must try the Pass amps, the X350.5 is supposed to be more liquid and pleasing, as well as the more current model. While the X-600s are arguably the best of the X series and really do sound great (I have a pair), it seems like you don't need the extra power.
Third, the only real way to make a determination of which of the three amps is best would be to audition them in your system. It would be real hard for me to be able to discern a difference between these three great amps without knowing how all the system synergies will work out. Is there any way you could try the Pass amps at no cost or minimal cost? (Of course, you could always buy them used then sell them for about the same price.)