Look up the Audiokarma "Official JD9 thread". It is a huge source of info for upgrade mods for the JD9. My JD9 II has upgraded caps, Burr Brown opamps, fuse, damping material and NOS RCA black plate tubes.
One of the good things about the JD9 is the excellent capacitance settings available for moving magnet carts. You can actually select a setting of zero if appropriate your cart. An example is a cart that has a capacitance spec of 100 picofarads. Well the phono cables will usually use up most or all of the "100" picofarads so, adding more with the preamp may not be desirable. A setting of zero could be the best choice in that case.