As I remember from having owned an OC9, the package included frequency measurement shows a steady rise from 15kHz-ish at an amount of around 3-4 dBs.
I have never tried the following, but many people complain about how bright the cart sounds:
I think a complementary lowpass filter consisting of a simple RC would help here. With an R of 100 ohms in series followed by a capacitor to ground between 50 and 100nF one would be in the ballpark, if I am not mistaken.
The OC9 is a very good cart, but to get it to sound substantial is a feat for many owners, judging from forum posts.